Synthesis of new rhodanine derivative molecules, their biological properties and sensor applications in environmental samples

Abstract

In this study, two new rhodanine derivative molecules (3 and 4) were synthesized, and the potentiometric sensor properties and certain biological functions of these novel molecules were investigated, following their characterization. The potentiometric performance characteristics of sensor 2 prepared using the molecule 3, and of sensor 10 prepared using the molecule 4 were further investigated under laboratory conditions. The sensors in which new rhodanine derivative molecules were used as ionophores were found to exhibit selectivity against Ba²⁺ ions among 19 different cations (Na⁺, Li⁺, K⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Co²⁺, Sr²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Cr³⁺, Al³⁺, Gd³⁺, Nd³⁺ and In³⁺). In the potential measurements performed using Ba²⁺ ion solutions, sensor 2 had a near–Nernstian response of 25.0 ± 2.5 mV/decade, and a detection limit of 7.66 × 10^–7 M in the concentration range of 1.0 × 10^–6 – 1.0 × 10^–1 M, while sensor 10 had a near–Nernstian response of 27.0 ± 2.3 mV/decade, and a detection limit of 9.77 × 10^–8 M in the concentration range of 1.0 × 10^–1 – 1.0 × 10^–7 M. Both sensors had a quick response time of 6 and 5 s, respectively; and sensor 10 was shown to be able work in a wider pH range (5.0–9.0) without being influenced by the changes in pH. Furthermore, both sensors exhibited a repeatable and stable behavior. The surface morphologies of the prepared potentiometric sensors were also examined by scanning electron microscope (SEM). Ba²⁺ ion analysis was performed in different environmental water samples using sensor 10, and the newly fabricated sensor was found to be able to determine Ba²⁺ ions with very high recoveries. These novel rhodanine derivative molecules were also observed to have a slight growth inhibitory potential against particular bacterial species in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13065-026-01780-7.

Introduction

Barium is an alkaline earth metal and 14th most commonly found element on the Earth's crust with a concentration of approximately 500 ppm [1]. Barium constitutes around 0.03% of the Earth's crust, mainly as the minerals “Barite” (also known as barytes or heavy spar) and “Witherite”. The abundance of barium is 13 μg/l in seawater. Based on its high chemical reactivity, it is not present as a free element (never found free in nature); but, found in the form of barium sulfate and barium carbonate. To date, barium has no known biological function in humans yet [2]. However, it was reported to be essential for the growth of some other organisms, including desmid green alga Closterium moniliferum which contain vesicles with barium sulfate in the crystalline form as barite [3]. Human uptake of barium is associated with the absorption mostly via food, and via drinking water to a lesser extent. Most food products generally contain lower than 3 mg/100 g barium, with the exception of Brazilian nuts which contain 150–300 mg/100 g of barium. Barium concentration in drinking water shows high variability, based on the geographical area, but it is generally present in the order of tens of micrograms per liter [4]. Bread consumption alone contributes to the daily intake of barium by diet with a 20%. In tissues, mean barium levels are estimated to be approximately 20 mg [5]. The concentration of barium present in food and drinking water generally is not high enough to be considered as a health risk for humans. However, data is still being collected to identify the health effects of long-term exposure to low concentrations of barium. People having the highest known risk of exposure to high levels of barium are those employed in industries making or using barium compounds.

Barium is used in various industrial applications [6, 7]. Barium salts (nitrate and sulfate) are present in numerous products, such as ceramics, plastics, adhesives, paints, bricks, tiles, glass, rubber, insect and rat poisons, electronics, drugs, vinyl stabilizers, railroad flares, fireworks, additives to lubricating oil, permanent magnets, fine chemicals, in drilling fluids for oils and gas wells, as a molecule giving green color in fireworks by burning barium nitrate and chlorate, as a white pigment in paints or to preserve old frescoes. Hair dyes containing barium are also sold in the market in multiple countries [8–10], and barium sulfate (BaSO₄) is permitted by the EU, and may be used as a colorant in different cosmetics and personal care products. Furthermore, research on novel materials designed to be used in medical, industrial, and technological applications resulted in the development of nanoparticles (NPs) based on BaSO₄ [11, 12] or on barium composites [13, 14]; thus, further increasing the possibility of environmental contamination by barium [15, 16].

Inductively coupled plasma mass spectrometry (ICP–MS, in short), atomic absorption spectroscopy (AAS) and other spectroscopic methods have been proposed previously to be used for the determination of barium concentration in different samples. However, these methods are not particularly suitable for on–site analysis, not easy to use, time-consuming, costly, and require experienced lab personnel and a well–equipped laboratory setting [17–20]. In contrast, potentiometric ion–selective electrodes (ISEs) have particular advantages including easy use, short response time, low limit of detection, wide concentration range, low cost, low–energy consumption, good selectivity and repeatability [21–23].

In the present study, we reported the development of novel Ba²⁺–selective potentiometric sensors using new rhodanine derivative molecules as ionophores, tested their analytical performances, applied them to the analysis of barium in different water samples. We also studied the growth inhibitory potential of these novel rhodanine derivatives against certain bacterial species.

Experimental

Chemicals and reagents

All reagents and solvents used in the synthesis of rhodanine derivative molecules were of high purity, and were obtained from Sigma-Aldrich and Merck. To prepare potentiometric sensors, high molecular weight poly(vinyl chloride) (PVC, density, 1.4 g/mL at 25 °C, powder), bis (2–ethylhexyl) adipate (DEHA, density 0.925 g/mL at 20 °C), di-2-ethylhexyl phthalate (DEHP, density 0.985 g/mL at 20 °C), dioctyl phenylphosphonate (DOPP, 0.967 g/mL at 25 °C), dibutyl phthalate (DBP) density 1.043 g/mL at 25 °C), o–nitrophenyloctylether (o–NPOE, density 1.04 g/mL at 25 °C), bis(2-ethylhexyl) sebacate (BEHS, density 0.914 g/mL at 25 °C), potassium tetrakis (p–chlorophenyl) borate (KTpClPB, ≥ 98.0%) and tetrahydrofuran (THF, anhydrous, density, 0.889 g/mL at 25 °C) were purchased from Sigma Aldrich. All metal nitrate salts (Ba²⁺, Na⁺, Li⁺, K⁺, Ca²⁺, Mg²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Co²⁺, Sr²⁺, Mn²⁺, Ni²⁺, Cd²⁺, Cr³⁺, Al³⁺, Gd³⁺, Nd³⁺ and In³⁺) used in selectivity studies were of analytical grade and were obtained from either Sigma Aldrich, Fluka and Merck. Ultradistiled water (18.2 MΩ cm⁻¹; MP Minipure water system, Dest up, 0513957) was used to prepare stock solutions using metal nitrate salts. Sodium hydroxide pellets (NaOH) and nitric acid (HNO₃) used in pH studies were purchased from Merck.

Apparatus

The ¹H–, ¹³C–NMR and FT–IR instruments used in the characterization studies of the newly synthesized ionophores (3 and 4) are Bruker Avance DPX–400 spectrometer and Jasco FT/IR 430 instrument, respectively. The melting points of these molecules were determined by Electrothermal–9100 apparatus. Surface images and energy–dispersive X–ray (EDX) analysis of developed sensors were carried out with Quanta FEG 450–FEI instrument. Potentiometric measurements (Medisen Medical Company, Türkiye) were made using a two-electrode system with the prepared sensors and an Ag/AgCl reference electrode (Thermo Scientific Orion 900,100).

Method

Synthesis of acyl aldehydes

The synthesis of acyl aldehydes was carried out as detailed in our previous study in the literature [24, 25]. Salicylaldehyde (1.22 g, 10 mmol) was dissolved in 50 mL of ethyl acetate and cooled in an ice bath. A solution containing the corresponding acyl chloride (10 mmol) and triethylamine (1.7 mL, 12 mmol) in 10 mL of ethyl acetate was added drop-wise under continuous stirring. After the addition was complete, the reaction mixture was allowed to warm to room temperature, and then stirred until complete conversion was confirmed by thin-layer chromatography (TLC). The reaction was quenched by the addition of 20 mL of water, and the aqueous layer was extracted with ethyl acetate (3 × 20 mL). The combined organic layers were dried over anhydrous sodium sulfate (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude product was obtained as a white solid and used directly in the next step without further purification.

Synthesis of novel rhodanine derivatives

Rhodanine 1 (1.5 mmol, 200 mg) and the appropriate aldehyde 2 (1.0 mmol) were dissolved in 10 mL of ethanol in a 50 mL round-bottom flask. Piperidine (50 mol%, 0.5 mmol, 50 µL) was added as a catalyst, and the reaction mixture was refluxed for 5 h. The progress of the reaction was monitored by thin-layer chromatography (TLC) using hexane/ethyl acetate (1:1, v/v) as the eluent. Upon completion, the temperature of the mixture was lowered down to room temperature, and the precipitated product was filtered, washed with cold ethanol and water, and dried under reduced pressure. The crude product was then recrystallized from ethanol to obtain the target compound in a good yield. ¹H–, ¹³C– NMR, FT–IR and HRMS spectra of the synthesized molecules are presented in supplementary files (Figure S1–S8).

( E )-2-((4-oxo-2-thioxothiazolidin-5-ylidene)methyl)phenyl 4-methoxybenzoate (3)

Yellow solid, yield 74%, mp 240–244 °C; R_f = 0.1 (EtOAc/hexanes = 7:3); ¹H NMR (400 MHz, DMSO–d₆) δ 13.86 (bs, 1H, NH), 8.12 (d, J = 8.8 Hz, 2H), 7.65–7.37 (m, 5H), 7.15 (d, J = 8.8 Hz, 2H), 3.87 (s, 3H). ¹³C NMR (100 MHz, DMSO-d₆) δ 200.9 (C = S), 174.6 (C = O), 169.5 (C = O), 169.3 (Ar–C), 155.5 (Ar–C), 137.6 (Ar-2CH), 137.4 (CH = C), 134.0 (Ar–CH), 133.5 (Ar–CH), 132.5 (Ar–CH), 131.4 (Ar–C), 129.3 (Ar–C), 129.1 (Ar–CH), 125.4 (C = C), 119.9 (Ar-2CH), 61.1 (OMe). FTIR (cm⁻¹) 3095, 2974, 2844, 1723, 1703, 1595, 1199, 1162, 835. HRMS (QTOF, ESI +) m/z: [M + Na]⁺ calc for C₁₈H₁₃NO₄S₂Na, 394.0184; found 394.0179.

( E )-2-((4-oxo-2-thioxothiazolidin-5-ylidene)methyl)phenyl 2,4-dichlorobenzoate (4)

Yellow solid, yield 78%, mp 245–249 °C; R_f = 0.2 (EtOAc/hexanes = 7:3); ¹H NMR (400 MHz, DMSO-d₆) δ 13.90 (bs, 1H, NH), 8.20 (d, J = 8.5 Hz, 1H), 7.93 (d, J = 1.9 Hz, 1H), 7.71 (dd, J = 8.5, 1.9 Hz, 1H), 7.64–7.54 (m, 5H). ¹³C NMR (100 MHz, DMSO–d₆) δ 196.4 (C = S), 170.3 (C = O), 163.0 (C = O), 149.9 (Ar–C), 139.0 (C = CH), 134.5 (Ar–C), 133.8 (Ar–CH), 132.4 (Ar–C), 131.3 (Ar–C), 129.5 (Ar–CH), 129.2 (Ar–CH), 128.5 (Ar–CH), 128.0 (Ar–CH), 127.5 (Ar–CH), 126.5 (Ar–C), 124.1 (Ar–CH), 123.9 (C = C). FTIR (cm⁻¹) 3143, 3042, 2845, 1748, 1692, 1582, 1209. HRMS (QTOF, ESI +) m/z: [M + Na]⁺ calc for C₁₇H₉Cl₂NO₃S₂Na, 431.9299; found 431.9293.

Preparation of rhodanine based sensors

In the current study, solid contact PVC membrane sensors were developed in two steps as detailed in our previous studies [26, 27]. First, the solid contact electrodes were prepared by dissolving graphite (50.0%), epoxy (35.0%) and hardener (15.0%) in approximately 3 mL of THF, and mixing until a certain viscosity was reached. Then, copper wires with one open end, approximately 15 cm in length, were dipped into this mixture several times, and then the coated copper wires were left to dry for 24 h at room conditions. Then, polymer membrane sensors were prepared, in which two different rhodanine derivative molecules (3 and 4) were used as ionophores. For this purpose, ionophore, plasticizer, lipophilic additive (KTpClPB) and PVC were dissolved in approximately 3 mL of THF in the proportions given in Table 1, and mixed until a certain viscosity was obtained. The previously prepared solid contact electrodes were dipped into the PVC membrane mixture several times, and their surfaces were coated. The coated electrodes were left to dry for 12 h at room conditions. The potentiometric performance properties of the sensors prepared in the ratios given in Table 1 were tested in laboratory conditions, and the performance properties of the sensors with optimum working conditions where these molecules (3 and 4) were used as ionophores were examined in further detail. Potential readings were systematically recorded using an Ag/AgCl reference electrode, ensuring a stable and reproducible reference point for all measurements. The temperature was rigorously maintained at a constant 25 ± 1.0 °C throughout the experiments to minimize thermal fluctuations that could affect electrochemical responses. The electrochemical cell configuration employed for these measurements was meticulously set up as follows:

Table 1.

PVC membrane potentiometric sensors prepared with newly synthesized rhodanine derivatives in this study, and their certain potentiometric performance properties

Sensor No	Ionophore	Membrane composition (w/w)									Potentiometric performance
		Ionophore	PVC	Plasticizers						KTpClPB	Linear range, M	Detection limit, M	Slope (mV/decade)	R2
				DEHA	BEHS	DBP	DOPP	o–NPOE	DEHP
1	3	32.0	3.0	64.0						1.0	1.0 × 10–5–1.0 × 10–1	7.40 × 10–6	19.5 ± 0.8	0.9982
2		32.0	3.0		64.0					1.0	1.0 × 10–6–1.0 × 10–1	7.66 × 10–7	25.0 ± 2.5	0.9972
3		32.0	3.0			64.0				1.0	1.0 × 10–5–1.0 × 10–3	9.24 × 10–6	14.5 ± 4.5	0.9689
4		32.0	3.0				64.0			1.0	1.0 × 10–3–1.0 × 10–1	2.40 × 10–4	25.1 ± 2.0	0.9612
5		32.0	3.0					64.0		1.0	1.0 × 10–5–1.0 × 10–1	9.79 × 10–6	22.0 ± 2.1	0.9963
6		32.0	3.0						64.0	1.0	1.0 × 10–5–1.0 × 10–1	9.43 × 10–6	23.8 ± 3.6	0.9912
7		32.0	2.0		65.0					1.0	1.0 × 10–5–1.0 × 10–1	8.99 × 10–6	19.0 ± 4.9	0.9771
8		32.0	4.0		63.0					1.0	1.0 × 10–5–1.0 × 10–1	8.89 × 10–6	20.6 ± 2.9	0.9918
9	4	32.0	3.0	64.0						1.0	1.0 × 10–5–1.0 × 10–1	8.91 × 10–6	19.2 ± 3.3	0.9953
10		32.0	3.0		64.0					1.0	1.0 × 10–7–1.0 × 10–1	9.77 × 10–8	27.0 ± 2.3	0.9977
11		32.0	3.0			64.0				1.0	1.0 × 10–5–1.0 × 10–1	6.98 × 10–6	20.5 ± 3.4	0.9943
12		32.0	3.0				64.0			1.0	1.0 × 10–4–1.0 × 10–1	2.41 × 10–5	20.0 ± 4.0	0.9869
13		32.0	3.0					64.0		1.0	1.0 × 10–5–1.0 × 10–1	9.66 × 10–5	14.3 ± 2.0	0.9871
14		32.0	3.0						64.0	1.0	1.0 × 10–4–1.0 × 10–1	2.0 × 10–5	29.6 ± 2.8	0.9819
15		32.0	2.0		65.0					1.0	1.0 × 10–5–1.0 × 10–1	9.40 × 10–6	24.3 ± 4.1	0.9905
16		32.0	4.0		63.0					1.0	1.0 × 10–5–1.0 × 10–1	7.86 × 10–6	25.5 ± 1.1	0.9953

Ag/AgCl; KCl (3 M) ║Ba²⁺ sample solution│ Ba²⁺ selective membrane│solid contact│Cu wire.

This configuration allowed for precise measurement of the potential difference generated by the interaction between the Ba²⁺ ions and the selective sensor membrane, relative to the stable Ag/AgCl reference electrode. All potential measurements were performed in triplicate.

For potentiometric measurements, the relationship between the concentration and the potential is governed by the Nernst equation, where E represents the cell potential observed at zero current.

Here, E is the potential (V), E° is the standard potential, n is the number of electrons, and a_i is the activity of the primary ion. R, T, and F are known constants. The sensitivity of a potentiometric sensor is known to be limited to 59.2/z mV at room temperature, the slope predicted from the Nernst equation, where z is the charge of the analyte ion. The potentiometric response of the Ba²⁺–selective electrode was described by the Nernst equation:

Since the analyte charge is 2 (z = 2), the mathematical relationship between the measured potential (mV) and the analyte concentration is:

Disk diffusion method for the analysis of bacterial growth inhibitory potentials of novel molecules

To examine the growth inhibitory potential of the newly synthesized rhodanine derivative molecules against six different selected bacterial species, a disk diffusion microbial susceptibility tests using these new molecules were performed according to the procedure used in our previous study [21].

Results and discussion

Characterization of novel rhodanine derivatives

As part of the present study, two novel 5-arylidene rhodanine derivatives were successfully synthesized. Following the synthesis protocol outlined in Scheme 1, the target compounds 3 and 4 were obtained via a piperidine-catalyzed Knoevenagel condensation between rhodanine and the corresponding aldehyde derivatives designed for this work. The reactions proceeded smoothly, producing the desired molecules in satisfactory yields. ¹H NMR analysis of compound 3 revealed a characteristic broad singlet at δ 13.86 ppm, corresponding to the NH proton of the rhodanine moiety. Aromatic and vinylic protons were observed in the region of δ 8.20–7.15 ppm, exhibiting splitting patterns consistent with their respective chemical environments and proton counts. In addition, the methoxy protons present in compound 3 appeared as singlets at δ 3.87 ppm. In the ¹³C NMR spectra, the C=S carbon of the rhodanine ring resonated at δ 200.9 ppm in compound 3; while in compound 4, it appeared slightly upfield at δ 196.4 ppm. The carbonyl (C=O) signals were observed in the range of δ 174.6–163.0 ppm. Vinyl and aromatic carbons gave resonances between δ 169.3 and 119.9 ppm, consistent with the expected number of carbon atoms. Additionally, the methoxy carbon in compound 4 appeared in the aliphatic region at δ 61.1 ppm. FT–IR spectral analysis further supported the structural assignments. The N–H stretching vibrations were observed between 3095 and 3143 cm⁻¹. Aromatic C–H stretching bands appeared in the range of 2974–3042 cm⁻¹, while aliphatic C–H stretching bands were detected at 2844–2845 cm⁻¹. The ester C=O stretching vibrations were observed between 1723 and 1748 cm⁻¹ in both compounds, whereas the C=O stretching bands of the rhodanine moiety appeared at 1692–1703 cm⁻¹. Conjugated C=C stretching vibrations were also recorded in the range of 1592–1595 cm⁻¹.

Scheme 1.

Synthesis of 5-Arylidenerhodanine derivatives via Knoevenagel condensation

Membrane composition

In the current study, two newly synthesized rhodanine-derived molecules were utilized as ionophores, and 16 sensors with different compositions were prepared in total. The synthesized rhodanine derivatives exhibited selectivity towards Ba²⁺ ions over different cationic species. Ion-selective sensors are analytical systems that can identify a specific ion (analyte) with high selectivity in a complex medium. This selectivity is achieved thanks to molecules called ionophores located in the active phase of the sensor. The ionophore has the ability to reversibly and selectively complex with the target ion and carry it to the membrane phase, thus enabling the ion to pass into the membrane; this ion distribution creates a phase-boundary potential at the membrane surface. In potentiometric measurement, the potential difference between the reference electrode and the ion-selective electrode originates from this phase-boundary potential, and is directly dependent on the analyte ion activity. The enhanced sensing performance of molecule 4 relative to molecule 3 can be attributed to the electron-withdrawing 2,4-dichloro substituents, which increase the polarization of the rhodanine-ester framework and strengthen interactions with Ba²⁺ ions. In contrast, the electron-donating methoxy group in molecule 3 reduces binding efficiency, leading to lower sensitivity. These electronic effects provide a clear structure–performance relationship for the observed sensor behavior. The compositions and potentiometric performance properties of the prepared sensors are given in Table 1. The detection limit values given in Table 1 were calculated by substituting the potential value corresponding to the point where the horizontal and vertical axes of the calibration curves intersect in the linear equation according to the rules stated by IUPAC [28]. Among the sensors with the same composition, the sensor prepared with molecule 4 (sensor 10) was found to have a broader concentration range and lower detection limit than the sensor prepared with molecule 3 (sensor 2). The behavior of the electrodes that do not contain the synthesized rhodanine derivatives (3 and 4) in their composition against Ba²⁺ ions is given in Fig. 1. As shown in this figure, sensors containing synthesized rhodanine derivatives do not exhibit a significant potential response in the concentration range of 1.0 × 10⁻⁵ to 1.0 × 10⁻¹ M

Fig. 1.

The effect of ionophores on sensor behavior

Potentiometric response and calibration curve

The potential responses and calibration curves of the sensors prepared using the newly synthesized rhodanine-derived molecules as ionophores are given in Fig. 2a–d. According to Fig. 2, the sensor 10 prepared with molecule 4 had a linear working range of 1.0 × 10^–7–1.0 × 10^–1 M (R² = 0.9977), and a detection limit of 9.77 × 10^–8 M, while the sensor 2 prepared with molecule 3 had a linear working range of 1.0 × 10^–6 –1.0 × 10^–1 M (R² = 0.9972), and a detection limit of 7.66 × 10^–7 M. Sensor 2 had a near–Nernstian response of 25.0 ± 2.5 mV/decade, whereas sensor 10 had a Nernstian response of 27.0 ± 2.3 mV/decade. As a result, sensor 10 prepared using molecule 4 as ionophore had an optimum composition (3.0% ionophore (4), 32.0% PVC, 64.0%BEHS and 1.0% KTpClPB), and is selective towards Ba²⁺ ions.

Fig. 2.

a, b Potentiometric response and calibration curve of sensor 2. c, d Potentiometric response and calibration curve of sensor 10

Repeatability

The repeatability of a sensor is a crucial indicator of its precision and reliability in providing consistent measurements under identical conditions. To rigorously assess this parameter, the optimized rhodanine-based potentiometric sensor was subjected to repeated measurements of Ba²⁺ solutions at three different concentrations: namely, 10^–2, 10^–3 and 10^–4 M. These concentrations represent the higher and middle range of the sensor's linear detection capability, providing a comprehensive evaluation of its performance. The repeatability plot and repeatability data of sensors 2 and 10 prepared with molecules 3 and 4, respectively, are given in Fig. 3a, b and Table 2. When these data was analyzed, the sensors prepared with the optimum components exhibited a repeatable behavior in the studied concentration range (10^–2, 10^–3 and 10^–4 M). According to Table 2, the newly prepared optimum sensor 2 was found to have lower standard deviations than sensor 10 at three different concentrations, respectively, of ± 1.32, ± 0.34, and ± 1.13.

Fig. 3.

a, b Repeatability analysis of sensors 2 and 10 against Ba²⁺ solutions of three different concentrations

Table 2.

Repeatability data of sensors 2 and 10 against Ba²⁺ solutions with different concentrations

Sensor	Concentration, M	Potentials, mV
		I	II	III	Average (± SD)
2	1.0 × 10–2	1216.75	1219.94	1218.75	1218.48 (± 1.32)
	1.0 × 10–3	1193.56	1193.50	1192.81	1193.29 (± 0.34)
	1.0 × 10–4	1164.38	1166.81	1166.75	1165.89 (± 1.13)
10	1.0 × 10–2	1201.19	1198.19	1198.5	1199.29 (± 1.35)
	1.0 × 10–3	1171.69	1170.19	1168.5	1170.12 (± 1.30)
	1.0 × 10–4	1141.69	1142.56	1138.69	1140.98 (± 1.65)

Potentiometric selectivity

Selectivity, which is undoubtedly one of the most important potentiometric performance properties of ion-selective sensors, is extremely critical in terms of determining the ionic species to which the prepared sensor is selective in the presence of different species. ISEs can of course respond to other species present in the environment; however, they should ideally be selective against only one species. The selectivity values of the sensors which were prepared with molecules 3 and 4 are given in Fig. 4a, b which show that the sensors prepared with the newly synthesized rhodanine derivative molecules exhibited very high selectivity against Ba²⁺ ions. The numerical expression of how well an ISE can differentiate the analyte ion from other interfering ions is determined with the help of selectivity coefficients. In the current study, the selectivity coefficients of the proposed sensor were calculated with the separate solution method of IUPAC [29]. The calculated selectivity coefficient values are shown in Table 3. When the data given in Fig. 4 and Table 3 are analyzed, it is clearly observed that the prepared sensors show high selectivity to Ba²⁺ ions.

Fig. 4.

Potentiometric selectivity of sensors developed with newly synthesized rhodanine derivatives a Sensor 2 b Sensor 10

Table 3.

Potentiometric selectivity of sensors 2 and 10 calculated according to IUPAC rules

Sensor 2			Sensor 10
Interfering ions	log		Interfering ions	log
Cr+3	–1.05	8.91 × 10–2	Na+	–1.69	2.04 × 10–2
Cu2+	–1.35	4.47 × 10–2	Cu2+	–1.72	1.91 × 10–2
Al3+	–1.93	1.17 × 10–2	Sr2+	–1.83	1.45 × 10–2
Na+	–2.09	8.13 × 10–3	Pb2+	–1.89	1.29 × 10–2
Li+	–2.13	7.41 × 10–3	Al3+	–1.93	1.17 × 10–2
Ca2+	–2.14	7.24 × 10–3	In3+	–2.06	8.71 × 10–3
Cd2+	–2.26	5.49 × 10–3	Cd2+	–2.16	6.92 × 10–3
Ni2+	–2.36	4.36 × 10–3	Ni2+	–2.23	5.89 × 10–3
Pb2+	–2.54	2.88 × 10–3	Co2+	–2.24	5.75 × 10–3
Zn2+	–2.57	2.69 × 10–3	Ca2+	–2.33	4.68 × 10–3
Gd3+	–2.60	2.51 × 10–3	Gd3+	–2.67	2.14 × 10–3
Sr2+	–2.70	1.99 × 10–3	Mg2+	–2.68	2.09 × 10–3
Mg2+	–2.77	1.70 × 10–3	Cr+3	–2.91	1.23 × 10–3
Nd3+	–2.94	1.15 × 10–3	Li+	–2.94	1.15 × 10–3
Mn2+	–2.97	1.07 × 10–3	Zn2+	–3.14	7.24 × 10–4
Co2+	–3.04	9.12 × 10–4	Nd3+	–3.28	5.25 × 10–4
In3+	–4.74	1.82 × 10–5	Mn2+	–3.58	2.63 × 10–4
K+	–5.51	3.10 × 10–6	K+	–4.05	8.91 × 10–5

Response time

Next, the response time of the prepared Ba²⁺–selective sensors was determined by considering the rules of IUPAC [28]. IUPAC defines the response time as 95% of the time required for the electrode to reach equilibrium for each tenfold concentration change, and is shown as t₉₅. For this purpose, the equilibrium time of the sensors during the transition from one concentration to another was determined. Based on data given in Fig. 5a, b, the response time of sensor 2 and sensor 10 was determined as 6 and 5 s, respectively.

Fig. 5.

The response time sensor 2 (a) and sensor 10 (b)

pH range

The pH range in which rhodanine-based sensors can work without being influenced by the changes in pH was determined using acid–base solutions prepared in the pH range of 2.0–12.0. The acidic pH (2.0–7.0) solution was prepared using HNO₃, the basic pH (8.0–12.0) solution was prepared using NaOH, and two different concentrations (10^–2 and 10^–3 M) of Ba²⁺ ion solutions were added to these solutions. The potential changes of both sensors for two different concentrations in the pH 2.0–12.0 range were then examined. The potential–pH graph was drawn with the obtained data to determine the pH working range of the sensors (Fig. 6a–b). According to Fig. 6a, b, sensor 2 prepared with molecule 3 was observed to be able to work without being influenced by changes in pH in the range of pH 5.0–8.0 in both concentrations, while sensor 10 prepared with molecule 4 could work without being affected by pH changes in the range of pH 5.0–9.0 in both concentrations. For prepared two sensors, the high potential values below this pH working range may be due to the presence of hydronium ions, while the low potential values above this range may be observed due to the formation of Ba(OH)₂ complexes.

Fig. 6.

pH working range of a sensor 2 and b sensor 10 at two different concentrations

Surface morphologies of the prepared sensors

Later in the study, the surfaces of the sensors with optimum composition prepared with two different rhodanine derivatives synthesized were studied using scanning electron microscopy. Before these analyses, some sensors were conditioned in 10^–2 M ion solutions for 30 min. Surface images and EDX spectra of conditioned and unconditioned sensors are given in Figs. 7 and 8 for sensors 2 and 10, respectively. While only PVC membrane components are present in the EDX spectrum of unconditioned sensors, the presence of barium ions is remarkable in the EDX spectrum of conditioned sensors. The white dots in the SEM images belong to Ba²⁺ ions, and it is clearly observed in the EDX spectrum of Figs. 7d and 8d that Ba²⁺ ions are attached to the sensor surface. Combined, based on the data from SEM/EDX analysis, it can be stated that Ba²⁺ ions can be attached to the cavities on the surface of the sensors, and the sensors can detect Ba²⁺ ions.

Fig. 7.

Sensor 2 in Ba²⁺ ion solution: a Unconditioned surface image, b EDX spectrum of the unconditioned surface, c Conditioned surface image after 30 min, d EDX spectrum of the conditioned surface

Fig. 8.

Sensor 10 in Ba²⁺ ion solution: a Unconditioned surface image, b EDX spectrum of the unconditioned surface, c Conditioned surface image after 30 min, d EDX spectrum of the conditioned surface

Analytical applications

In this study, when all the obtained data are evaluated, it was clearly seen that sensor 10 had better potentiometric performance compared to sensor 2. Therefore, Ba²⁺ ion concentrations in five different water samples were determined only using sensor 10. All water samples were collected in clean bottles, immediately stored at 4 °C, and analyzed within 24 h of collection to avoid any contamination or changes in ion concentration.

Ba²⁺ ions were added to the water samples at the rates given in Table 4, and subsequently potentiometric measurements were taken with the prepared sensor. The potential values obtained ​​were then written in the calibration curve, and the Ba²⁺ ion concentrations in each sample were calculated. According to the obtained data, the produced sensor can be proposed to be able to determine Ba²⁺ ions at high recovery rates (> 97.98).

Table 4.

Applications of the Ba²⁺–selective sensor in the analyses of real samples

Real Samples	Added Ba2+ (ppm)	Found Ba2+ concentration, ppm*	%Recovery
Purified water	56.0	54.87 (± 0.14)	97.98
Tap water (Tokat, Turkey)		55.31 (± 0.57)	98.77
Spring water		55.46 (± 0.57)	99.04
Snow water		55.56 (± 0.40)	98.78
Bottled water		55.57 (± 0.21)	98.52

^*Average value (n = 3)

Comparison study

The sensors reported in the present study were compared with other sensors previously reported in the literature with respect to certain parameters: linear working range, detection limit, Nernstian response, pH working range and response time (Table 5). Based on the data given in Table 5, fabricated sensor 10 was found to have the widest linear working range, the lowest detection limit and the fastest response time relative to its counterparts in the literature. The data we obtained showed that the synthesized new molecule 4 exhibited a very successful performance in the determination of Ba²⁺ ions when compared to other studies in the literature.

Table 5.

Comparison of the prepared sensor to the other Ba²⁺–selective potentiometric sensors previously reported

Ionophore	Linear range, M	Limit of detection, M	Slope, mV dec−1	pH working range	Response time, s	References
Thiosemicarbazone derivative molecule	1.0 × 10−6–1.0 × 10−1	5.54 × 10–7	28.9 ± 1.43	6.0–9.0	< 10	[30]
Dimethyl-4,4-dimethoxy-5,6,5,6-dimethylene dioxy biphenyl-2,2-dicarboxylate	1.0 × 10–5–1.0 × 10–1	5.0 × 10–6	Not reported	4.0–9.0	Not reported	[31]
2,3,4-pyridine-1,3,5,7,12-pentaazacyclopentadeca-3-ene	1.41 × 10−6–1.0 × 10−1	Not reported	30.0	2.5–7.0	18	[32]
4–4’-methylenediantipyrine	1.0 × 10–6–1.0 × 10–2	5.2 × 10–7	29.7 ± 0.3	3.4–10.6	15	[33]
Barium(II)–Rose Bengal	5.0 × 10–5–1.0 × 10–1	2.5 × 10–6	28.5 ± 0.4	4.5–10.0	20	[34]
3-deoxy-D-erythro-hexos-2-ulose bis (thiosemicarbazone)	1.0 × 10–6–1.0 × 10–2	5.6 × 10–7	29.6 ± 0.5	2.6–11.0	15	[35]
4-aminobenzoic acid 2-diethylaminoethyl ester	1.0 × 10–5–1.0 × 10–1	4.23 × 10–6	Not reported	5.0–11.0	8	[36]
Dibenzo-24-crown-8	1.0 × 10–6–1.0 × 10–3	6.1 × 10–7	30.1	4.1–9.0	< 10	[37]
1,2-bis(diarylphosphine oxide) naphthalene	1.0 × 10–5–1.0 × 10–1	5.0 × 10–6	30.0	3.0–11.0	60	[38]
macrocyclic tetrabasic ligand	3.6 × 10–6–1.0 × 10–1	1.9 × 10–6	29.7 ± 0.2	2.5–7.5	10	[39]
(E)-2-((4-oxo-2-thioxothiazolidin-5-ylidene)methyl)phenyl 4-methoxybenzoate (3)	1.0 × 10–6–1.0 × 10–1	7.66 × 10–7	25.0 ± 2.5	5.0–8.0	6	This work
(E)-2-((4-oxo-2-thioxothiazolidin-5-ylidene)methyl)phenyl 2,4-dichlorobenzoate (4)	1.0 × 10–7–1.0 × 10–1	9.77 × 10–8	27.0 ± 2.3	5.0–9.0	5	This work

Growth inhibitory potential of the novel rhodanine derivative molecules against certain bacterial species

The antimicrobial test results of the newly synthesized rhodanine derivative molecules are interpreted in detail in the supplementary files.

Conclusion

In the current study, two new molecules derived from rhodanine were synthesized for the first time, and their sensor properties and certain biological activities were subsequently analyzed. A total of 16 sensors with different compositions were prepared, and the potentiometric performance characteristics of the sensors prepared with these two different molecules providing optimum conditions were compared with each other. Sensor 10 prepared with molecule 4 showed a better performance compared to sensor 2 prepared with molecule 3 in terms of particular performance properties such as working range, detection limit, response time and pH working range. The obtained results indicate that both synthesized rhodanine derivative molecules can be used as ionophores in the potentiometric analysis of Ba²⁺ ions; however, molecule 4 shows better sensor properties. These molecules were also found to have growth inhibitory potentials depending on bacterial species.

Abbreviations

SEM

Scanning Electron Microscope

EDX

Energy–dispersive X–ray spectroscopy

FTIR

Fourier Transform Infrared Spectroscopy

NMR

Nuclear Magnetic Resonance

Q-TOF (HRMS)

Quadrupole Time-of-Flight Mass Spectrometry

ISE

Ion Selective Electrode

PVC

Poly(vinyl chloride)

DEHA

Bis (2–ethylhexyl) adipate

DEHP

Di-2-ethylhexyl) phthalate

DOPP

Dioctyl phenylphosphonate

DBP

Dibutyl phthalate

BEHS

Bis(2-ethylhexyl) sebacate

o–NPOE

o–Nitrophenyl octyl ether

KTpClPB

Potassium tetrakis (p–chlorophenyl) borate

THF

Tetrahydrofuran

NaOH

Sodium hydroxide

HNO₃

Nitric acid

IUPAC

International Union of Pure and Applied Chemistry

Author contributions

Onur Cem Altunoluk: Investigation, Analysis, Oguz Özbek: Investigation, Analysis, Writing—Review&Editing, Erbay Kalay: Investigation, Analysis, Writing—Review&Editing, Caglar Berkel: Investigation, Analysis, Writing—Review&Editing, Ömer Isildak: Investigation, Analysis.

Funding

The authors thank the The Scientific and Technological Research Council of Turkey (TUBITAK) (Project Number: 124Z600) for its support. Onur Cem Altunoluk was supported financially by TUBITAK–BIDEB 2210/A Scholarship Program for M.Sc. students.

Data availability

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.