PdRuO2/PVP nanomaterial as a highly selective, stable, and applicable potentiometric sensor for the detection of Cr3+

PdRuO2/PVP nanomaterial was synthesized using a straightforward method and characterized using advanced analytical methods such as TEM, XRD, XPS, elemental mapping and SEM. The synthesized PdRuO2/PVP nanomaterial was used as an ionophore in potentiometric sensor electrodes and successfully adapted to Cr3+ ion detection in a large number of aqueous samples. Several experimental parameters of the PdRuO2/PVP sensor such as potentiometric behavior, selectivity, repeatability, response time, pH, titration, and recovery in real samples were investigated. Potentiometric behavioral characteristics were performed in the concentration range 1 × 10−6–1.0 × 10−1 M. The repeated experiments performed six times showed that there was no deviation in the measurements. The limit of detection of the PdRuO2/PVP potentiometric sensor was very low with a value of 8.6 × 10−8 M. The potentiometric measurements showed that the synthesized PdRuO2/PVP ionophore was highly effective in detecting Cr3+ in a wide pH range of 2.0–8.0 and was found to have a shelf life of over 1 year. As a result, the synthesized PdRuO2/PVP electrode material was found to be highly selective, stable, and applicable for Cr3+ detection. Graphical Abstract


Introduction
Chromium is used in many industrial activities and is involved in the structural activities of living organisms.Generally, chromium is found in nature in the structure of compounds at Cr(VI) and Cr(III) oxidation levels [1,2].Because of its many uses such as wood painting, leather layering, metallurgy, chrome plating, spray painting, and electroplating, chromium is released into the environment as wastewater because of industrial activities [3,4].Being a heavy metal, the release of chromium into the environment causes metal pollution.Also, as an essential metal ion for living organisms, Cr(III) plays a vital role in the metabolic reaction of structural molecules of carbohydrates, fat, and protein.The absence of Cr(III) in the human body causes some illnesses such as diabetes, sugar metabolic disorder, cataracts, cardiovascular disease, and blindness [1].Also, intake excessively of Cr(III) results in several disorders such as vomiting, cancer, diarrhea, nausea, and bleeding in the human body.Therefore detection of Cr(III) in different liquid samples is very important [5].So far, several methods like colorimetric, chemiluminescence, coupled plasma spectroscopy, fluorescence resonance, X-ray fluorescence, atomic absorption spectrometry (AAS), high-performance liquid chromatography (HPLC) have been used for the detection of Cr(III) [6,7].Despite the wide usage of the mentioned methods, they have many disadvantages such as laborious sample preparation, cost, and the need for special equipment.Despite that, potentiometric sensors are efficient, environmentally compatible, low-cost, and versatile.Therefore, potentiometric sensors are seen as an alternative to these methods [4,[8][9][10].Additionally, compared to the common techniques, chemical sensors have gained considerable importance due to having several advantages such as easy sample preparation, wide analysis range, simple device use, and good selectivity [11,12].Therefore, preparing a suitable electrode material is crucial to the sensor's activity.
One of the problems encountered in electrode development is that the material used clumps in the solvent and binds poorly to the electrode.This weakens the modification in the resulting electrode and limits the application of the electrode.To overcome this problem, it is necessary to use a material with high solubility.In other words, it is necessary to use a building material that can easily disperse in the solvent environment and form a film on the electrode surface.Polyvinylpyrrolidone (PVP) is a polymeric substance that is highly soluble, easily dispersed, film-forming, non-toxic, and strongly adhesive [13].
Accurate determination of the amount of Cr(III) found in many biological systems and industrial samples is essential because of its widespread availability in many sources.Therefore, the development of biosensors that measure selective, fast, and sensitive chromium facilitates the detection of ions like chromium [14,15].To date, various sensors have been prepared for different purposes using different materials [16].In some studies, some electrode materials have been developed for the detection of various drugs in samples [17,18].In addition, there are many different fields of study such as nanocomposite materials, wearable device production, thin film development, and antioxidant activity [19][20][21][22][23].In some studies, a synergistic effect was created by loading materials such as Fe and MIL-100 on the PVP surface.With this effect, materials with high surface area, high electronic conductivity, and high electrocatalytic properties have been obtained [24].
Modifying relatively cheaper materials with precious metals (Pd, Ru, Pt, Re) can improve the response and selectivity of the resulting sensor to the target substance.Ruthenium (Ru)-based sensors made with PVP and sensitive to Hg +2 and Ag +1 ions in an EDTA environment were developed by Zhao et al. [25].In these studies, Ru-supported PVP nanoparticles were obtained homogeneously and the simultaneous detection of these two ions was successfully achieved.In a study, Pd-doped CeO 2 nanoparticles were prepared and used as gas sensors in a methanol environment, and different amounts of gas were detected successfully by keeping the Pd ratio at 3% [26].As mentioned above, one of the biggest problems in synthesizing potentiometry sensor materials is ensuring that the synthesized material is dispersed in a suitable solvent environment and adheres to the electrode surface.Although it is possible to prepare homogeneously distributed materials, it is very difficult to detect the desired target molecule or ion accurately.To overcome existing problems, it is necessary to choose a suitable material that can detect the desired target ion.Generally, advanced analytical methods such as TEM, XRD, and SEM are used to reveal the chemical and morphological properties of the materials used in the structure of the electrode.In this study, PdRuO 2 /PVP nanomaterials were synthesized using the reduction/impregnation method, their morphological and structural properties were revealed with some advanced analytical methods and were used effectively in the detection of Cr(III).The nanocluster of Pd and Ru metals on the supporting material (PVP) was investigated by TEM analysis.The crystalline structure was revealed by XRD analysis and surface/content properties were investigated by SEM-EDS.Here, for the first time, PdRuO 2 /PVP nanomaterials were synthesized using a simple method and used as potentiometric sensors.For the first time, PdRuO 2 /PVP nanomaterial was applied successfully for the detection of Cr(III) in sensor studies.

Instruments
TEM analyses were performed with a TEM Hitachi HT7800 device capable of scanning the elemental range of Boron-Uranium (5B-92 U) with a voltage of 120 kV.XRD analyses were carried out with a Panalytical Zetium brand device.SEM-EDS analyses were performed with a Hitachi Regulus 8230 model device.All potentiometric measurements of Cr 3+ were performed with a computercontrolled Medisen brand potentiometric sensor device.A glass Ag/AgCl electrode containing saturated 3 M KCl was used as the reference electrode.The elemental mapping analyses were performed by Hitachi Regulus 8230 operating in a range of 0.5-30 kV, a resolution of 1 kV in 0.9.The XPS analyses were implemented by X-ray photoelectron Spectrometer (XPS) device operating at 2 min 35 total acquisition time, Al K Alpha source gun type, 300 µm spot size.

PdRuO 2 supported PVP nanoparticles synthesis (PdRuO 2 /PVP)
The synthesis of PdRuO 2 /PVP nanocomposite was carried out using the reduction impregnation method as stated in the literature [27], summarized as follows.Pd(NO 3 ) 2 hydrate and RuCl 3 hydrate were used as precursors for the synthesis.2% metal material of ruthenium and palladium and 200 mg PVP as a support material in the synthesis.Firstly, a solution containing PVP (200 mg) and 2% Pb-Ru was prepared in 5-mL distilled water.Then, the resulting mixture was stirred at room conditions and 600 rpm for 2 h.Then, sodium boron hydride, 15 times the mole of metal used, was added to the resulting mixture.After reduction is achieved, the solid part is filtered and washed with plenty of pure water allowed to dry at 120 °C, and then stored for use as electrode material.

Preparation of PdRuO 2 /PVP nanocomposites-based electrode
Firstly, PdRuO 2 /PVP was prepared in a membrane form to apply to the surface of the electrode.For this, a 100-mg mixture containing ionophores (PdRuO 2 /PVP), graphite, PVC, epoxy, and hardener components was prepared to be used in the composition of the electrodes.Approximately 3 mL of THF was added to the composite components prepared in different proportions and mixed until a homogeneous mixture was obtained.The composites in THF were vortexed to homogenize and mixed for approximately 1-2 min.After ensuring the appropriate viscosity, PdRuO 2 /PVP membrane sensors were prepared by dipping the tip parts of the copper wires prepared the night before into the composites.The prepared biosensors were dried in a dark environment for approximately 12-15 h and kept in a dark environment for taking measurements.The prepared membranes were applied to the surface of the electrodes and left for 12 h to dry.The dried electrodes were kept in 10 −3 M Cr 3+ solution for 1 h to condition.Solutions that were not used in subsequent studies were kept in room conditions and a dark environment.Table 1 shows the composition electrode used in the detection of Cr 3+ ions.Ionophores with different properties were prepared by taking different amounts of substances.The obtained ionophore and R 2 values are given in Table 1.As can be seen from the obtained values, the PdRuO 2 /PVP-based ionophore we prepared gave the best R 2 value.

Potentiometric sensor measurements of Cr 3+
All experiments were carried out at room temperature, 10 −2 M solution concentration, 1000-1800 Volt potentiometer common parameters.The experimental setup used in the measurements is given in Fig. 1.Measurements were carried out by bringing the reference electrode and working electrodes to the same level in Cr 3+ solutions of various concentrations (10 −1 -10 −10 M).In order not to disrupt other solution concentrations in subsequent measurements, the electrode used after the measurements was thoroughly washed with deionized water and dried.Care was taken to ensure that there was no residue from the previous measurement during the measurements.

XRD analyses
XRD analysis results of PdRuO 2 /PVP material are given in Fig. 2. As stated in previous studies [28]; the peaks observed at 40.25°, 46.8°, 68.23°, and 82.2° correspond to the (111), (200), (220), and (311) planes for Pd.A peak of 11.92° corresponds to 002 of C present in PVP.Additionally, the peaks seen in the range 21-29° are due to the presence of carbon in the structure of PVP.The mean crystallin particle size was calculated using the Scherrer equation (D = k * λ / β * cos(θ)) and found to be 17.93 nm.The 29.09° seen in the XRD analysis is very close to the specific value of 28.8° for Ru-O [29].This value indicates the presence of Ru in the structure of PdRuO 2 /PVP.JCPDS card number of XRD analyses of PdRuO 2 /PVP material was found to be 96-154-8201.Figure 3 shows the TEM analysis results of the PdRuO 2 / PVP material in the 50-200-nm range.It is seen that PdRuO 2 metal and metal oxides generally have a spherical appearance.The diameters of the spherical atom clusters seen in TEM analyses were measured diagonally.The diameters of the atomic clusters found in TEM analyses were measured from two directions.The obtained values were transferred to the ImageJ program.Then, the average particle size of this value was plotted using the Orgine Lab program.The calculation made using TEM analysis images showed that the mean particle size of PdRuO 2 /PVP material was 16.85 nm.This value obtained is very close to the crystal particle size.
SEM analyses were performed to examine the surface structure of the obtained PdRuO 2 /PVP electrode material, and SEM analysis images at different scales are given in Fig. 4. As can be seen from the figures, some porous structures are present in the surface structure.As the surface structure is examined more closely, the pores become clearer in some places.In general, the surface structure has a flat ground appearance.
EDS analyses were performed to examine the chemical composition of the obtained PdRuO 2 /PVP electrode material.As can be seen from the EDS analysis results in Fig. 5, the material obtained contains palladium and ruthenium metals in its structure.It is seen that it contains C and N, which originate from the PVP structure.
Elemental mapping analysis was performed to visualize the elemental composition of the PdRuO2/PVP nanomaterial we synthesized.The results of the elemental mapping XPS analyses were performed to obtain information about the oxidation states of the elements in the PdRuO 2 /PVP material and the chemical environment of these elements.XPS analysis results of PdRuO 2 /PVP material are given in Fig. 7.In the XPS analysis of Ru 3d given in Fig. 7b, the binding energies of 282 and 285.5 eV correspond to 3d5 and 3d3 of Ru 4+ [30].As can be seen in Fig. 7d, there is no visible impurity due to the element added to the catalyst.O 1 s XPS analyses (Fig. 7c) show that the binding energies of oxygen are at 532 and 531 eV.The presence of RuO 2 in the material structure increased the power at the O 1 s peaks [31].
The binding energies of 335-336.5 eV, which are characteristic of metallic Pd, are seen in Fig. 7a.The 342 value deviated slightly from 340 eV, which is specific for metallic Pd.Such situations can be observed during XPS analysis because of oxidation or Pd interactions with RuO 2 .Such results have also been observed in other studies [32][33][34].

Detection limit, linear range, and slope
Potentiometric measurements of Cr 3+ selective PdRuO 2 /PVP electrodes were carried out in 1.10 −6 -1.10 −2 M Cr 3+ solutions.The results of the potentiometric measurements obtained and the calibration curves created using these measurements are given in Fig. 8.As a result of the measurements obtained with PdRuO 2 /PVP electrodes in 1.10 −6 -1.10 −2 M tamoxifen solutions, a linear graph was formed and the slope of this graph was found to be 20.6 ± 0.2 mV/slope Fig. 8d.In addition, the detection limit was expanded by taking more measurements with 1.10 −7 -1.10 −10 M solutions (Fig. 8a).Thus, the detection limit was obtained more accurately.Using the intersections in the two parts of the extrapolation in the calibration plot given elsewhere, the detection limit was found to be 8.3 ± 0.4.× 10 −8 M. Figure 8c shows the results of experiments at 10 −2 -10 −6 M that show the potentiometric behavior.As seen, a linear potentiometric behavior was obtained with the synthesized sensor.

Response time
The response time was determined to find the time for the constant potential value of the Cr 3+ ion of the PdRuO 2 /PVPbased sensor to reach equilibrium.To understand this, the response time parameter was measured by dipping the electrode in a 10 −2 M solution into the next 10 −3 M solution and then into a 10 −4 M solution.As stated in the literature, the time when 95% of the potential value reached equilibrium  8b.As can be seen from the experimental results, the obtained electrode reaches equilibrium quickly and the response time was found to be 5 s.

Selectivity of PdRuO 2 /PVP biosensor
Selectivity in biosensors is found by measuring the response of the electrodes obtained to the target species in the presence of different species in a solution.In the study, the selectivity of PdRuO 2 /PVP electrodes towards Cr 3+ in 1.0 × 10 −2 -1.0 × 10 −6 mol.L −1 solutions was measured.Using Eq. (1) [35], the selectivity coefficients of PdRuO 2 / PVP electrodes against Cr 3+ were calculated according to the E A = E B condition [36].Potential values of measurements conducted with solutions of different species at concentrations of 1.0 × 10 −2 mol.L −1 were found and the obtained potential values are calculated using Eq. ( 1) given below; (1)  where, E A = activity of chromium ion, E B = activity of interfering ion, Z A = charge of chromium ion, Z B = charge of interfering ion, R = ideal gas constant, T = temperature (K) and F = Faraday constant.The selectivity coefficient of PdRuO 2 /PVP electrodes towards different species is given in Table 2.As can be seen from the results, the selectivity of the PdRuO 2 /PVP electrode towards Cr 3+ is quite high compared to other ions.
Measurements carried out with PdRuO 2 /PVP potentiometric sensors showed that the Cr 3+ ion is 103 times more selective than interfering Li 1+ and Hg 2+ ions and 108 times more selective than the farthest interfering Ca 2+ ion.As can be seen from the data in Table 2, the PdRuO 2 /PVPbased sensor showed a potentiometric effect only against Cr.When other results were examined, it was revealed that the

Reusability and lifespan of PdRuO 2 /PVP electrodes
The experiments of the reusability and lifetime of PdRuO 2 / PVP electrodes were performed in 1.0 × 10 −2 M, 1.0 × 10 −3 , and 1.0 × 10 −4 M Cr 3+ M Cr 3+ solution.The reusability results of PdRuO 2 /PVP electrodes are given in Fig. 9a.Eight repeated measurements were taken in each solution.The average potential values and standard deviations obtained in 1.0 × 10 −2 M, 1.0 × 10 −3 and 1.0 × 10 −4 M Cr 3+ M solutions were found to be 1606.2± 0.9 mV, 1588.2 ± 1.9 mV, and 558.4 ± 0.8 mV, respectively.As can be seen from the results, the repeatability of PdRuO 2 /PVP electrodes is quite good.These results show that there is almost no change in the measurement efficiency of the PdRuO 2 /PVP electrode after 8 measurements.The lifetime of PdRuO 2 /PVP was obtained in standard prepared solutions by daily calibration and by calculating the slopes in linear ranges.The obtained values indicate that the lifespan of the prepared electrodes exceeds 1 year.It was observed that there was almost no change (less than 2%) in the calibration curves obtained for PdRuO 2 /PVP electrodes over 1 year.Figure 10 depicts the test results of the sensor that was kept for 1 year.As can be seen from the results, it was determined that the obtained sensor had a very stable structure.
1.0 × 10 -2 mol.L −1 Cr 3+ solutions were used to detect the response of PdRuO 2 /PVP potentiometric electrodes to Cr 3+ at different pH values.pH ranges were adjusted to 1.0-14.0pH using NaOH and HCl.The findings are given in Fig. 9b and seen in the results of experiments conducted with different solutions at various pHs the potentiometric electrodes give constant pH values in the range of 2.0-8.0.

Potentiometric titration experiments using PdRuO 2 / PVP electrodes
PdRuO 2 /PVP potentiometric biosensors exhibited selectivity behavior against Cr 3+ ions in potentiometric titration experiments having 1.0 × 10 −2 mol L −1 Cr 3+ , 1.0 × 10 −2 mol.L −1 EDTA solutions at room temperatures.In real samples, PdRuO 2 /PVP electrodes along with Cr 3+ ions were used as indicators.The end point of titration experiments was detected by observation of the decreasing Cr 3+ concentration Fig. 9c.In the titration experiments of Cr 3+ with the PdRuO 2 /PVP potentiometric sensor, the sensor exhibited an end value of 25.6 mL of Cr 3+ concentration.A sharp turning point and a standard sigmoid shape were obtained in the resulting titration curves.This shows that the titration results obtained can be used in the determination of Cr 3+ .

Effectiveness of PdRuO 2 /PVP electrodes
To detect the effectiveness of our developed PdRuO 2 /PVP electrodes, some results of electrodes developed in the literature for the detection of Cr 3+ ions were investigated as seen in Table 3. Table 3 compares the analytical response characteristics of the chromium(III) ion-selective sensor we designed with those of many previously reported Cr 3+   Analytical applications of the obtained PdRuO 2 /PVP electrode, which is selective for Cr 3+ , were implemented with different drinks.For this purpose, a certain amount of Cr 3+ ion was added to known beverage samples, and potentiometric measurements were performed.The measurement results obtained are given in Table 4.The concentrations of Cr 3+ were calculated by substituting the obtained potential values into the equation in the linear graph.The concentrations found were compared with the Cr 3+ concentrations added to beverages.These compared values revealed a high amount of recovery and showed that the PdRuO 2 /PVP electrode obtained was an alternative method to traditional analytical methods used for Cr 3+ ion detection.
The statistical analysis results of the data of measurements of Cr(III) detection with PdRuO 2 /PVP-based sensor are given in Table 5. Six measurements were performed with solutions having 0.1, 0.01, and 0.0001 M concentrations, and their results were added to the table, and their average and standard deviation were calculated using Excell packet program.The standard deviation value found in statistical calculations is quite low.This value shows that the electrode is quite stable and these results are compatible with the results obtained in repeated measurements.

Conclusion
In this study, for the first time, PdRuO 2 /PVP electrode material was successfully synthesized, its chemical structure was elucidated using various advanced analytical methods and was successfully used as a potentiometric sensor in the detection of Cr 3+ ion.The developed PdRuO 2 /PVP potentiometric sensor was found to have outstanding selectivity towards Cr 3+ ions among a wide range of interfering ions.PdRuO 2 /PVP electrode was simply and efficiently used as a suitable sensing element (ionophore) as a selective potentiometric sensor.The sensor is highly sensitive with a detection limit of 8.6 × 10 −8 mol.L −1 for the rapid detection of chromium(III) ions in the range of 1 × 10 −6 -1.0 × 10 −1 M in aqueous samples.The easy structure and selective and fast response of the developed detection device enable PdRuO 2 /PVP to be used in the routine analysis of various real aqueous matrices.To the best of our knowledge, the developed PdRuO 2 /PVP sensor is one of the most selective and sensitive potentiometric sensors reported so far for the rapid and accurate detection of ultra-trace amounts of Cr 3+ in industrial, environmental, and biological samples.The features such as linear range, detection limit, selectivity, operating pH range, and lifetime clearly showed that the proposed PdRuO 2 /PVP-based sensor can be classified as the best Cr 3+ sensor among the reported sensors, especially in terms of high selectivity and least interfering ion.Also, having strong repeatability and 1 year lifetime revealed that PdRuO 2 /PVP electrode material is extremely suitable for the detection of Cr 3+ ions in liquid samples.
Funding Open access funding provided by the Scientific and Technological Research Council of Türkiye (TÜBİTAK).This study was partially supported by Yuzuncu Yil University Scientific Research Projects Support Unit, project numbered FYL 2023 10672.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.

Declarations
Conflict of interest The authors declare no competing interests.
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Fig. 8
Fig. 8 Potentiometric experimental results; limit of detection (a), response time (b), potentiometric behavior (c), and slope of potential

Table 1
The content of PdRuO 2 /PVP based-electrodes, detection limit values, and R 2 data

Table 3
Some experimental results of electrodes developed for the detection of Cr 3+ in literature

Table 4
The potentiometric measurement results were performed with different beverages for the detection of Cr 3+ While the response time of previously developed sensors to Cr 3+ was between 10 and 20 s, the response time of PdRuO 2 /PVP sensors is only 5 s.Compared to previous studies, the developed sensor can detect Cr 3+ ions at much lower limits and in a wider pH range.Also, the sensor gives very good values in comparisons such as detection limit and concentration range.

Table 5
The statistical analysis of the data of measurements of Cr(III) detection with PdRuO 2 /PVP-based sensor