Using Polyvinyl Chloride and Screen-Printed Electrodes for the Determination of Levofloxacin in the Presence of Its Main Photo-Degradants in River Water: A Comparative Study

Abstract

The application of membrane sensors for the detection and quantification of pharmaceutical environmental contaminants has become a significant goal in recent years. Due to the widespread application of levofloxacin hemihydrate (LEVO) in medicine, its occurrence in the environment, especially in surface water bodies like rivers, is quite likely. Extended exposure of river water to sunlight and the photo-degradability of LEVO may facilitate its photo-degradation. To measure LEVO in the presence of its principal photo-degradants, two sensitive and selective membrane electrodes were designed. A polyvinyl chloride electrode (PVCE) and a screen-printed electrode (SPE) were constructed for the selective analysis of the investigated drug. Phosphomolybdic acid was used to prepare a lipophilic ion pair with the studied drug. All test parameters were optimized to achieve the best electrochemical performance. The electrodes demonstrated a linear range from 1 × 10⁻⁶ M to 1 × 10⁻² M. The PVCE and SPE demonstrated slopes of 55.80 ± 0.70 mV/decade and 56.90 ± 0.50 mV/decade, respectively. The aforementioned sensors demonstrated satisfactory performance within a pH range of 3.0 to 5.0. The fabricated sensors were successfully utilized to accurately quantify LEVO in the presence of its primary photo-degradants. The membranes were effectively utilized to measure LEVO in river water samples without requiring pre-treatment processes.

1. Introduction

In contemporary society, characterized by advanced technology, life-threatening infections caused by microbes remain prevalent. These conditions are managed with antibiotics, particularly a well-established class of synthetic antibacterial agents known as fluoroquinolones, which are extensively used in human therapy, veterinary medicine, and breeding industries [1].

Levofloxacin hemihydrate (LEVO) is chemically denoted as (S)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid hemihydrate, as illustrated in Figure 1. It is the l-isomer of the racemate ofloxacin, an antimicrobial agent that is a member of the fluoroquinolone group. It is a zwitterion that exhibits exceptional solubility in both acidic and basic media due to the presence of a carboxylic acid group at the 3-position and a basic piperazinyl ring at the 7-position [2].

LEVO exhibits efficacy against the majority of Gram-negative, Gram-positive, and anaerobic bacteria [3]. While LEVO is effective in treating severe infections, its overuse has resulted in antibiotic resistance and accumulation in aquatic ecosystems, posing significant risks to both ecological integrity and human health due to the emergence of drug-resistant bacteria [4]. Consequently, it is imperative to employ extremely sensitive and effective methods to monitor the levels of LEVO in aquatic environments. Similarly, these approaches must selectively identify LEVO even in the presence of structurally analogous compounds, particularly its primary photo-degradation products, which are likely to be found in surface waters such as rivers due to the high susceptibility of LEVO to photo-degradation.

Photo-stability of pharmaceuticals pertains to the influence of light on the stability of active ingredients and/or finished formulations. Consequently, photo-stability assesses the impact of light on the stability of pharmaceutical compounds. Light-sensitive pharmaceuticals can be influenced by both natural and artificial light sources. Sunlight can induce interactions between the drug and endogenous substances, converting the medication into hazardous byproducts that may lead to the formation of various oxygenated free radicals. A minimal amount of light exposure can significantly impact photo-sensitive medications. The generation of phototoxic compounds ultimately impacts the effectiveness of LEVO as a photo-sensitive therapeutic agent. Its therapeutic efficacy is concurrently reduced by photo-degradation [5].

LEVO is recognized as a photo-sensitive medication. It experiences rapid degradation when exposed to near-UV light, resulting in a succession of photo-degradation products, all of which are analogs modified at the N-methyl piperazine moiety of LEVO. The primary identified photo-degradants of levofloxacin are desmethyl levofloxacin (LEVO-DES) and levofloxacin N-oxide (LEVO-OXD) [6]. Figure 1 illustrates the chemical formulas of LEVO and its primary photo-degradants.

The selective determination of the investigated medication amidst its photo-degradants is crucial for the accurate measurement of LEVO, given the significant structural similarity between the drug and its primary photo-degradants.

Due to its significant role as potent antibiotic, LEVO has been analyzed using many techniques, including high-performance liquid chromatography (HPLC) [7,8], capillary electrophoresis [9,10], and spectrophotometry [11,12]. A variety of analytical methods were utilized to assess the stability of the drug under research in the presence of its various products of degradation. Liquid chromatography was used to quantify the presence of the thermal, photolytic, and hydrolytic degradation products of LEVO in both bulk and dosage forms [13,14].

Many of these analytical procedures are costly, time-consuming, cumbersome for routine use, necessitate skilled personnel, involve sample manipulation, extraction, and derivatization stages, and are unsuitable for colorful and turbid solutions [15]. Conversely, electrochemical techniques, particularly potentiometric membrane sensors, offer straightforward and economical options for selective and sensitive analysis, presenting a generally safe and ecologically friendly technology with reduced reagent usage [16].

Ion-selective electrodes (ISEs) are uncomplicated membrane-based potentiometric sensors that are capable of accurately and precisely measuring ion activity. LEVO was quantified utilizing various membrane sensors, including molecularly imprinted, polyvinyl chloride, carbon paste, and screen-printed membrane sensors [17,18,19]. However, all reported methods employ the fabricated electrodes for the determination of the analyzed drug in bulk, dosage form, or biological fluids.

A thorough literature analysis revealed that there are currently no publications regarding the potentiometric photo-stability assessment of the investigated medication in the presence of its principal photo-degradants in river water.

The primary objective of this study is to fabricate, optimize, and validate polyvinyl chloride electrodes (PVCEs) and screen-printed electrodes (SPEs) for the quantification of LEVO in the presence of its principal photo-degradants, and to employ the developed sensors for the accurate measurement of LEVO in river water samples.

2. Materials and Methods

2.1. Instrumentation

Jenway (London, UK) supplied the potentiometer (type 3510) used for the potentiometric measurements. A reference electrode (Ag⁰/AgCl) was included with it. All pH measurements were taken using a Jenway pH meter.

2.2. Chemicals, Reagents, and Standard Solutions

Cayman Chemical of Ann Arbor, MI, USA, supplied the LEVO, LEVO-DES, as a hydrochloride salt, and LEVO-OXD, with corresponding purities of 100.34%, 100.55%, and 100.22%. Prolabo (Nantes, France) supplied a chemical set that included sodium hydroxide (NaOH), hydrochloric acid (HCl), boric acid, acetic acid, phosphoric acid, tetrahydrofurane (THF), potassium chloride (KCl), sodium chloride (NaCl), and phosphomolybdic acid (PMA). Sigma of Darmstadt, Germany, on the other hand, supplied the following: dioctyl phthalate (DOP), sodium tetrakis(trifluoromethyl)phenyl borate (STFPB), polyvinyl chloride (PVC), cyclohexanone, acetone, graphite, and tricresyl phosphate (TCP). Aquatron water stills provided the deionized water.

To prepare a 1 × 10⁻² M stock standard solution of LEVO-Cl, an accurate weight of LEVO (0.37 gm) was weighed and transferred to a 100 mL volumetric flask. Complete dissolution was performed in 20 mL of 0.01 M HCl and the volume was completed using the same solvent. Working standard solutions were made by meticulously combining the stock solution with the deionized water. The concentrations of these solutions ranged from 1 × 10⁻³ to 1 × 10⁻⁸ M.

KCl/HCl buffer (pH 1) was prepared by mixing 100 mL of 0.2 M KCl and 268 mL of 0.2 M HCl. On the other hand, Britton–Robinson buffer (BRB) (pH 2–8) was prepared by mixing equal volumes of 0.04 M boric acid, 0.04 M o-phosphoric acid, and 0.04 M acetic acid. The mixture was then titrated with 0.2 M NaOH to the desired pH [20].

2.3. Preparation of LEVO Ion Exchanger

A mixture of 75 mL of 0.01 M LEVO-Cl and 25 mL of 0.01 M PMA was used to prepare levofloxacinium–phosphomolybdate (LEVO-PM). After the precipitate formed, it was filtered and washed with deionized water to remove any extra chloride ions. It was then left to dry at room temperature and ground into a fine powder.

2.4. Preparation of Electrodes

To prepare the PVC membrane sensor, 10 mg of the ion pair complex (LEVO-PM) was combined with 10 mg of STFPB, 190 mg of PVC, and 400 mg of TCP in a 5 cm diameter Petri dish. Five mL of THF was added to facilitate the dissolving of the previous mixture. The solvent was eliminated by allowing the Petri plate to remain at room temperature for 24 h. A membrane with a diameter of about 8 mm and a thickness of about 0.1 mm was removed and affixed to an interchangeable PVC tip using THF, which was connected to the end of the electrode’s body. An internal reference solution was prepared by mixing equal quantities of LEVO-Cl standard solution and NaCl, both at a molarity of 1 × 10⁻² M. The internal reference electrode was a 1 mm diameter Ag⁰/AgCl wire that was immersed in the internal reference solution. The constructed electrode was ultimately conditioned by immersing it in a 0.01 M LEVO-Cl solution for 24 h.

Conversely, the SPE (5 mm × 35 mm) was fabricated using a homemade carbon ink, which was manufactured by combining 0.08 g of LEVO-PM, 0.26 g of graphite powder, 10 mg of STFPB, 190 mg of DOP, and 0.47 g of PVC with a cyclohexanone–acetone mixture in a 1:1 ratio until achieving complete homogeneity. The ink was then applied to a PVC substrate (insulator) and subsequently covered with Teflon tape, leaving a rectangular section (5 mm × 5 mm) at both ends of the electrode for measurement and electrical contact [21]. The SPE was ultimately conditioned by immersion in a 0.01 M LVF-Cl solution for 24 h.

2.5. Sensors’ Calibration and Optimization

Following the conditioning step, the sensors were calibrated by immersing them in standard LEVO-Cl solutions with concentrations ranging from 1 × 10⁻² to 1 × 10⁻⁸ M (pH 4.0). Deionized water was employed to cleanse each membrane between measurements. Standard graphs were prepared. The graphs were employed to determine the unknown drug concentrations.

Optimal membrane performance was achieved by evaluating and modifying the items influencing membrane efficacy. The efficacy of the manufactured electrodes was assessed in accordance with IUPAC guidelines [22].

Two different plasticizers (TCP and DOP) with differing polarity were used to evaluate their effects on membrane performance. The aim was to determine the plasticizer that produced the most favorable performance of the manufactured electrodes.

The influence of pH on the efficacy of the synthesized membranes was examined by variating the pH in a range of 1–8 using KCl/HCl and BRB buffers, to a standard solution of LEVO-Cl at a concentration of 1 × 10⁻³ M. A graph was constructed to ascertain the ideal working pH by correlating the electrode potential with the pH of the solutions. The repeatability and potential stability of the manufactured sensors were examined over a one-month period. Daily, LEVO-Cl standard solutions with concentrations between 1 × 10⁻² M and 1 × 10⁻⁶ M were employed to assess the performance of the constructed sensors. Subsequently, the slope for each membrane was calculated in millivolts per decade. The estimated slopes were compared to the original ones.

The accuracy and precision of the suggested analytical method was validated by performing a statistical comparison between the results obtained from the analysis of pure LEVO-Cl samples using the fabricated sensors and a reference-reported HPLC method [14].

To evaluate membrane selectivity, the modified separate solution approach was used, which involved using the rearranged Nicolsky equation [23]. By studying the potential responses of the constructed electrodes in the presence of different interferents, such as inorganic ions (Na⁺, K⁺, and NH₄⁺) and structurally related fluoroquinolones (norfloxacin hydrochloride and ciprofloxacin hydrochloride), the potentiometric selectivity coefficient (PSC) for each interferent was calculated. The following formula was then used to calculate the PSCs:

Log K_D,B ^pot = ((E₁ − E₂)/S) + (1 + (z₁/z₂)) log a

E₁ stands for the potential measured in a solution containing 1 × 10⁻³ M of LEVO-Cl. The potential measured in a solution containing 1 × 10⁻³ M of the interfering chemical is denoted by E₂. z₁ is the charge of LEVO-Cl, while z₂ is the charge of the interfering species. The electrode calibration plot’s slope is denoted by S.

2.6. Application

2.6.1. Quantification of LEVO-Cl in the Presence of Its Main Photo-Degradants

Variegated amounts of intact LEVO-Cl (from 90% to 10%) and its primary photo-degradants (from 10% to 90%) were used to prepare laboratory-prepared combinations. A pH of four was set for the finished mixes. Using the calibration graphs, the potentials that resulted from submerging the membranes in the mixtures were used to calculate the LEVO-Cl concentration.

2.6.2. Determination of LEVO-Cl in Different Water Samples

A number of water samples, including distilled and tap water, were prepared using exact concentrations of LEVO-Cl. A pH of 4.0 was set for the prepared samples. The percentage of drug recovery was determined by measuring the drug concentrations using the manufactured sensors.

In Cairo, Egypt, three separate locations along the River Nile were sampled for water that did not contain LEVO. On the other hand, three river water samples suspected to contain LEVO were withdrawn. The samples were adjusted to a pH of 4.0. The samples were filtered using nylon filter to remove any trace of tiny particles. The filtered solutions were preserved in opaque glass vials to prevent the samples from photo-degradation.

The LEVO-Cl-free river water samples were treated with measured amounts of the pure LEVO-Cl, leading to drug concentrations of 1 × 10⁻³ M, 1 × 10⁻⁴ M, and 1 × 10⁻⁵ M. The fabricated membranes were used in conjunction with the standard curves to measure the LEVO-Cl concentrations, and the recovery percentage was computed using the drug concentrations that first spiked.

The built sensors were used to determine the LEVO concentrations in the real river water samples suspected to contain LEVO using the standard curves. A comparison was carried out between the results acquired by applying the suggested method and those obtained by using a reference-reported method [14] after applying solid-phase extraction.

3. Results

In this work, PVCEs and SPEs are fabricated. In these sensors, a potential is developed due to the movement of the ionic form of the studied drug across the fabricated membrane, leading to an equilibrium state across the fabricated membrane (Scheme 1) [18].

3.1. Evaluation and Validation of the Fabricated Sensors

Two membrane sensors (PVCE and SPE) were fabricated showing linear response in the concentration range 1 × 10⁻⁶ M and 1 × 10⁻² M, as shown in Figure 2.

Table 1. Electrochemical response characteristics of the fabricated electrodes.

Parameter	PVCE	SPE
Slope (mV/decade) *	55.80 ± 0.70	56.90 ± 0.50
Response time (s)	20	15
Working pH range	3.0–5.0	3.0–5.0
Concentration range (M)	1 × 10−6–1 × 10−2	1 × 10−6–1 × 10−2
Stability (days)	21	18
Accuracy (Mean * ± SD)	100.20 ± 0.81	100.42 ± 0.93
Detection limit (M)	5 × 10−7	5 × 10−7
Ruggedness †	101.41 * ± 1.32	100.27 * ± 1.11
Robustness Ψ	100.28 * ± 0.56	100.98 * ± 0.49

* Average results of five determinations. ^† Comparing the results with those obtained by different sensor assemblies using a Hanna digital ion-analyzer. ^Ψ Carried out by measuring different known LEVO-Cl concentrations in response to slight pH changes (pH 4 ± 0.2).

lists the evaluation criteria that demonstrate the efficacy of the fabricated electrodes. In accordance with the data in Table 1, the slopes of the PVCE and SPE were 55.80 ± 0.70 and 56.90 ± 0.50 mV/decade, respectively.

The fabricated membranes’ response time, optimal pH range, lifespan, effect of plasticizer type, and detection limit were all investigated in compliance with IUPAC guidelines [22].

The membrane components can significantly influence the characteristics of the produced sensors. In the case of PVCE, TCP plasticizer demonstrated superior membrane characteristics for proximity to the ideal Nernstian slope and rapid response time. The alternative plasticizer DOP exhibited a potential slope of 50.30 ± 0.80 mV/decade (n = 3) with a response time of 45 s (average response time for five concentrations from 1 × 10⁻⁶ M to 1 × 10⁻²). The superior performance observed with TCP usage is attributed to its higher dielectric constant, which enhances ion mobility and the ion-exchange process at both membrane interfaces [24]. When the additive STFPB was introduced to the electrodes, the response time improved, the membrane resistance decreased, and more ions could be exchanged, leading to an overall improvement in the electrode response [24]. Conversely, the SPE response improved with the incorporation of DOP as a plasticizer, likely attributable to the high lipophilicity of the ion pair LEVO-PM, which was utilized in a substantial quantity (80 mg). Consequently, a more lipophilic plasticizer is required to enhance the solubility of this ion pair to produce a non-rigid paste; however, a reduced quantity of the ion pair (10 mg) was employed in the case of the PVCE.

Assessment of the fabricated sensors’ sensitivity was carried out by finding the detection limit at the intersection of the standard curve linear segments [25]. The methods’ sensitivity was exceptional, as shown in Table 1, with detection limits as low as 5 × 10⁻⁷ M. With their exceptional sensitivity, the suggested sensors are well suited for use in environmental analysis. They also showed a quick response time. The proposed membranes can be used to reliably measure LEVO in real environmental samples (river water).

An essential factor influencing the performance of the manufactured sensors is the pH of the medium. In the pH range of 3.0 to 5.0, the manufactured electrodes clearly showed a nearly constant potential response. Figure 3 shows that the developed sensors can be used to reliably determine LEVO-Cl in this pH range.

According to the estimates, the PVCE has a lifetime of 21 days and the SPE of 18 days. Plotting the calibration graphs on a daily basis and periodically assessing their properties allowed for the estimation of the fabricated sensors’ lifetime (Figure 4).

A key aspect that has a big impact on the membrane evaluation process is the response time. Details on how long it takes for a manufactured sensor to stabilize and keep its potential value constant (at 90% of the ultimate equilibrium potential) after a tenfold increase in concentration are given [22]. Twenty seconds accounted for the response time of the fabricated PVCE and fifteen seconds for the SPE. The acquired response times reflect the facile ion-exchange process across the fabricated membranes.

The analytical assay’s procedure was assessed for its robustness by examining its resistance to deliberate changes in working conditions, including minor pH variations. To confirm the procedure’s reproducibility, its ruggedness was also evaluated. Table 1 presents a clear summary of these factors. In addition, the assessment of assay accuracy was conducted by utilizing the fabricated sensors to analyze LEVO-Cl samples with specific concentrations and the results were compared with those obtained by analyzing the same LEVO-Cl samples using a reported HPLC method [14]. The statistical comparison of the obtained results is clarified in

Table 2. Statistical comparison between the determination of pure LEVO-Cl by the fabricated membranes and the reference-reported method.

Item	PVCE	SPE	Reference Method [14] *
(Mean ± SD)	100.20 ± 0.81	100.42 ± 0.93	99.81 ± 0.59
RSD	0.81	0.93	0.59
Variance	0.65	0.86	0.35
n	5	5	5
F-value (6.39)	1.90	2.50	-
Student’s t-test (2.306)	0.920	1.242	-

* RP-HPLC method using C18 column (4 µm, 25 cm × 4.6 mm, i.d.) as stationary phase, and mobile phase composed of a mixture of 0.5% (v/v) triethyl amine in sodium dihydrogen orthophosphate dihydrate (25 mM; pH 6.0) and methanol using a simple linear gradient. The detection was carried out at 294 nm. Values in parentheses are the theoretical values of t and F at p = 0.05.

.

The modified separate solution method [23] was utilized to assess the selectivity of the constructed membranes. The objective was accomplished by testing a range of inorganic interferents, including Na⁺, K⁺, and NH₄⁺, along with structurally similar interferents such as norfloxacin hydrochloride and ciprofloxacin hydrochloride. The potentiometric selectivity coefficient (PSC) was subsequently calculated for each interferent.

Table 3. Potentiometric selectivity coefficients of the proposed CPZ-selective sensors by the modified separate solution method.

Interferent	PVCE (Mean * ± S.D.)	SPE (Mean * ± S.D.)
Na+	2.3 × 10−3 ± 0.54	2.2 × 10−3 ± 0.62
K+	3.6 × 10−4 ± 0.55	3.5 × 10−4 ± 0.55
NH4+	3.4 × 10−4 ± 0.67	3.3 × 10−4 ± 0.74
Norfloxacin hydrochloride	2.8 × 10−4 ± 0.89	2.9 × 10−4 ± 0.59
Ciprofloxacin hydrochloride	3.5 × 10−4 ± 0.64	3.6 × 10−4 ± 0.78

* Average of five measurements.

displays the calculated PSCs. The low values of the estimated PSCs can confirm the method’s exceptional selectivity.

3.2. Method Application

3.2.1. Determination of LEVO-Cl in the Presence of Its Main Photo-Degradation Products

The method’s selectivity was examined in the presence of up to 90% of LEVO-Cl’s main photo-degradants. The fabricated sensors were used to analyze a variety of lab-prepared mixtures with concentrations of degradants ranging from 1 × 10⁻⁴ M to 9 × 10⁻⁴ M and concentrations of LEVO-Cl ranging from 9 × 10⁻⁴ M to 1 × 10⁻⁴ M. The data presented in

Table 4. Results obtained for the analysis of laboratory-prepared mixtures containing different ratios of intact LEVO-Cl and its main photo-degradation products (LEVO-DES and LEVO-OXD) using the fabricated sensors.

Added Intact LEVO-Cl	Added LEVO-DES	Added LEVO-OXD	PVCE (Intact Drug Recovery% * ± S.D.)	SPE (Intact Drug Recovery% * ± S.D.)
90% (9 × 10−4 M)	5%	5%	100.23 ± 0.67	101.33 ± 0.45
70% (7 × 10−4 M)	15%	15%	100.13 ± 0.45	99.43 ± 0.35
50% (5 × 10−4 M)	25%	25%	102.43 ± 0.45	99.98 ± 0.56
30% (3 × 10−4 M)	35%	35%	100.31 ± 0.57	101.56 ± 0.65
10% (1 × 10−4 M)	45%	45%	101.16 ± 0.45	101.14 ± 0.45

* Average of three measurements.

illustrate the specificity of the proposed technique and confirm its ability to accurately measure the specified drug, even in the presence of up to 90% degradation products.

3.2.2. Assay of LEVO-Cl in Different Water Samples

In both distilled and tap water samples to which the studied drug was added, the sensors were successfully used to determine the quantity of LEVO-Cl. Additionally, a number of LEVO-Cl-free river water samples and samples loaded with known amounts of LEVO-Cl were examined using the fabricated sensors. This was implemented in order to confirm how well the sensors worked in different water matrices. The calculated recovery percentages are shown in

Table 5. Determination of LEVO-Cl in spiked river water samples using the fabricated membrane sensors.

Specimen	PVCE (Rec.% * ± S.D.)	SPE (Rec.% * ± S.D.)
Distilled water	100.89 ± 0.45	102.03 ± 0.82
Tap water	100.45 ± 0.78	100.45 ± 0.67
River water sample 1	99.46 ± 0.68	100.01 ± 0.87
River water sample 2	102.34 ± 1.56	101.31 ± 0.54
River water sample 3	100.76 ± 0.97	101.56 ± 0.54

* Average of five measurements.

.

On the other hand, three real river water samples suspected to contain LEVO were assayed by the fabricated membranes. The results obtained were compared to those obtained using a reference-reported method [14]. The results are declared in

Table 6. Determination of LEVO in real river water samples.

Sample Number	PVCE Conc. β (M) ± S.D.	SPE Conc. β (M) ± S.D.	Reference Method [14] * Conc. β (M) ± S.D.
Sample 1	2.10 × 10−4 ± 0.65	2.12 × 10−4 ± 0.78	2.11 × 10−4 ± 0.45
Sample 2	3.32 × 10−5 ± 0.45	3.30 × 10−5 ± 0.86	3.29 × 10−5 ± 0.89
Sample 3	5.47 × 10−4 ± 0.72	5.45 × 10−4 ± 0.59	5.48 × 10−4 ± 0.42

^β Average of three measurements; * RP-HPLC method using C18 column (4 µm, 25 cm × 4.6 mm, i.d.) as stationary phase, and mobile phase composed of a mixture of 0.5% (v/v) triethyl amine in sodium dihydrogen orthophosphate dihydrate (25 mM; pH 6.0) and methanol using a simple linear gradient. The detection was carried out at 294 nm.

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4. Discussion

Due to LEVO’s extensive usage across diverse human activities, its presence in the environment, especially in surface waters such as rivers, is likely to happen more often. The river water’s exposure to the natural UV light coming from the sun and the photo-lability of LEVO may enhance its photo-degradation. From this perspective, the fabrication of membrane electrodes that exhibit sensitivity to LEVO is an essential objective for monitoring, evaluating, and quantifying LEVO in the presence of its principal photo-degradants. Due to its affordability, ease of use, superior sensitivity, absence of sample pre-processing, and straightforward scalability, potentiometric membrane sensors have been optimally utilized in recent years [16].

The research demonstrates the proficient application of PVCEs and SPEs for the analysis of LEVO samples, utilizing PMA as an ion-pairing agent. The voltage measured by the developed sensors is attributed to the ion-exchange process across the fabricated membranes, which is exactly proportional to the concentration of LEVO in the solution.

This study comprises the development of two membrane electrodes capable of selectively detecting and quantifying LEVO in river water samples, despite the presence of its principal photo-degradants (LEVO-DES and LEVO-OXD), which exhibit significant structural resemblance to LEVO. The primary benefit of these electrodes is their ability to conduct this analysis without requiring any sample pre-processing, distinguishing them from other analytical procedures such as HPLC and LC-MS/MS.

The performance of the membranes was significantly influenced by the characteristics of their components. Concerning the PVCE, TCP was the superior plasticizer compared to DOP due to its enhanced capacity for ion exchange at both membrane interfaces. Conversely, DOP was the chosen plasticizer for SPE because of its higher lipophilicity, which aligns with the elevated lipophilicity of the ion pair (LEVO-PM), resulting in the production of a less rigid paste.

The detection limit of the constructed membranes demonstrates considerable sensitivity aligned with their intended application. Moreover, they exhibited a rapid response time throughout a broad concentration range.

The pH is a critical characteristic that influences membrane function and their capacity to properly detect the examined medication. The manufactured sensors demonstrated a consistent response within a pH range of 3.0 to 5.0. Below this pH range, the manufactured sensors are unlikely to be utilized due to the presence of LEVO as a dicationic species. Additionally, the positively charged protons may disrupt the functionality of the sensors. To ensure the best performance of the sensors, it is not recommended to use them above pH 5 since LEVO may be present as a zwitterionic species [19].

Careful validation of the proposed method’s selectivity is necessary to ensure its efficacy, even with various interferents in real samples. The suggested method’s selectivity is affected by the sample ion’s lipophilicity, the interferent ion on the sample side, and its three-dimensional (3D) structure. A receptor’s selectivity in an ion exchanger is affected by its 3D structure. The rate of ion exchange across surfaces is accelerated when lipophilic ionic sites are present. Because they allow for the rapid passage of ions and guarantee that the manufactured membrane has desirable physical characteristics, plasticizers are essential for membrane sensors [25].

The synthesized electrodes were efficiently utilized for the quantification of LEVO-Cl, even in the presence of up to 90% main degradation products. Furthermore, the designed sensors were utilized for the selective detection and quantification of the investigated medication in various water samples containing LEVO-Cl, without requiring any sample preparation.

LEVO-Cl was previously determined by the fabrication of membrane-sensitive electrodes. Abdel-Haleem et al. [18] quantified the studied drug in its dosage forms, spiked urine, and serum without considering the drug stability. On the other hand, the work cited in this article comprises the determination of LEVO-Cl in spiked and real river water samples in the presence of its main photo-degradation products. In addition, the fabricated membranes illustrated in this article have better sensitivity as demonstrated by the lower detection limit. Also, Abdel Ghani et al. [19] fabricated polyvinyl, carbon paste, and screen-printed electrodes solely to study the effect of pH on the electrochemical behavior of LEVO-Cl.

5. Conclusions

The work’s novelty arises from the lack of previous studies on the potentiometric membrane quantification of the target drug in the presence of its major photo-degradants, commonly seen in river water carrying LEVO. The common presence of the LEVO photo-degradation products in the sample matrix can be attributed to the high vulnerability of the studied drug to photo-degradation. The presence of LEVO photo-degradation products in river water samples containing LEVO poses a significant challenge for its quantification, owing to the considerable chemical similarity between LEVO and its principal photo-degradants. The absence of a sample pre-treatment step illustrates the method’s superiority compared to the spectroscopic and chromatographic techniques utilized for LEVO analysis. Furthermore, the suggested approach is more economical than HPLC and LC-MS/MS owing to its lower costs per sample.

Both electrodes were well equipped for their function, demonstrating satisfactory selectivity even in the presence of a wide range of inorganic interferents and structurally similar interferents such as norfloxacin hydrochloride and ciprofloxacin hydrochloride. Also, the fabricated sensors exhibited a linear response in a wide concentration range (1 × 10⁻⁶ M to 1 × 10⁻² M). At the same time, the synthesized sensors demonstrated excellent sensitivity, as shown by their detection limit (5 × 10⁻⁷ M). The SPE exhibited a faster response compared with the PVCE. Conversely, the PVCE had a longer lifespan in comparison to the SPE.