Optical Ammonia-Sensing Probe Based on Surface-Plasmon Resonance of Silver-Nanoparticle-Decorated Superparamagnetic Dendritic Nanoparticles

Ammonia is a serious contaminant of aquaculture water due to its continuous release into the water environment during the biological processes of aquatic animals. Ammonia accumulation in water has negative environmental impacts, including eutrophication and the death of aquatic organisms. Therefore, sensitive and accurate determination of ammonia is an urgent need, especially in pisciculture systems. Here, we report the fabrication of a novel magnetic–hyperbranched nanomaterial-based ammonia-sensing probe for the fast and sensitive determination of ammonia in water. The proposed probe is composed of poly(amidoamine) (PAMAM)-coated superparamagnetic iron oxide nanoparticles (SPIONs) decorated with silver nanoparticles. Changing the ammonia concentration is associated with a corresponding change in the surface plasmon resonance property of silver nanoparticles. The proposed nanosystem was characterized with FTIR spectroscopy, SEM imaging, energy-dispersive X-ray (EDX) analysis, TEM imaging, X-ray diffractometry (XRD), and vibrating sample magnetometry (VSM). The TEM images showed a homogenous and uniform distribution of the nanoparticles with an average nanoparticle size of 200 nm, while the surface silver nanoparticles have an average particle size of 10–50 nm. The proposed optical ammonia sensor was successfully used to determine the concentration of ammonia in water samples by measuring the change in the solution absorbance at 428 nm. The obtained results revealed high recovery values (96.3–104.7%) and very low detection (LOD = 5.69 mg/L) and quantification (LOQ = 18.96 mg/L) limits. The standard plot is linear in the concentration range of 10–50 mg/L with an r2 value of 0.9980. Sandell’s sensitivity of the most promising sensor (NP-III) among the investigated systems was found to be 0.15 µg/cm2, which indicates high sensitivity.


Introduction
Nitrogen plays a crucial role in aquatic ecosystems because it is considered one of the very important nutrients for sustaining aquatic life. Ammonia can enter aquatic environments through anthropogenic discharges such as agricultural runoff and municipal effluents. The introduction of large amounts of ammonia via these routes may have severe, irreversible influences on aquatic ecosystems. Ammonia is present in one of two species depending on the pH, unionized (ammonia, NH 3 ) and cationic ammonia (ammonium, NH 4 + ). At pH < 8.75, ammonium is the predominant species, whereas ammonia predominates when the water pH is above 9.75 [1]. It is thought that un-ionized ammonia is the reason behind ammonia toxicity; un-ionized ammonia has adverse effects on fish growth, organ weight, hematocrit, and gill conditions [2]. Despite the lower toxicity of ammonium compared to ammonia, high concentrations of ammonium are also toxic because they hinder the outflow of ammonia from the gills. This constitutes a serious threat to the life of fish populations and other aquatic animals, which also has a significant negative impact on human health and economic development. Therefore, continuous monitoring of ammonia concentration in water is very crucial, thus dictating the development of novel, highly sensitive, simple, and rapid ammonia sensing methods.
Nanotechnology involves the study, development, manufacture, and processing of nanoscale materials, and then applying them in various fields such as medicine, agriculture, sensors, and engineering. Nanomaterials (materials whose size is less than 100 nm) show different characteristic physicochemical properties due to changes in their quantum mechanical features. Properties such as extensive high surface area-to-volume ratio, extremely high reactivity, enhanced mechanical properties, improved solubility, and unique optical properties provide important advantages enabling the application of nanomaterials in a plethora of research areas [3][4][5][6]. For instance, the application of nanomaterials in sensors (e.g., optical and electrochemical sensors) allows for lower material and energy consumption, as well as reducing environmental hazards. Among the different types of sensors, optical sensors have significant benefits over traditional analytical approaches because they enable direct, real-time, fast, and simple detection/determination of a wide range of biological, environmental, and pharmaceutical compounds. Their advantages include excellent specificity, sensitivity, compact size, and low cost. In the last decade, non-invasive optical detections have progressed from fluorescence spectroscopy to surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS) [3].
Recently, researchers have become interested in superparamagnetic nanomaterials that demonstrate distinguished magnetic properties because the magnetic moment of the entire crystallite tends to align in one direction with the applied external magnetic field, resulting in a greater net magnetic moment than typical paramagnetic materials. An example of superparamagnetic materials is magnetite (Fe 3 O 4 ), an iron oxide that can be synthesized in a very small size and extremely crystalline forms. Superparamagnetic iron oxide nanoparticles (SPIONs) are attractive nanomaterials due to their flexible surface chemistry, high expression of surface charges, and their flexibility in being coated with various organic or inorganic compounds [7]. When an external magnetic field is applied to SPIONs, they become magnetized to saturation and then lose any residual magnetic interaction once the magnetic field is withdrawn [7]. SPIONs can be coated with hyperbranched polymers (HBP), which exhibit a high expression of surface functional groups available for chemical interactions/bonding. Due to the flexible surface functionality of SPIONs and their large surface area of interaction, which enable more expression of the coating polymer on their surface, SPIONs were selected to be the base of the ammonia sensing probe. Moreover, the excellent magnetic response of SPIONs makes them more controllable while suspended in the solution in terms of dispersion and the possibility of collection upon applying an external magnetic field after finishing the assay procedure.
Polyethyleneimine (PEI) is a polymer made up of an amino group and a two-carbon aliphatic CH 2 -rH 2 spacer. The branched polymer can be made up of several layers of branches making a tree-like structure (dendrimer) such as poly(amidoamine) (PAMAM). Coating of SPIONs with PAMAM provides an enormous number of surface amino groups available for chemical interaction compared to the basic PEI structure. These surface sites can be used as reducing functional groups for the deposition of metallic nanomaterials (e.g., silver nanoparticles, AgNPs), thus permitting a wider range of applications [8].
In the last decade, different optical ammonia-sensing approaches have been used for determining the concentration of ammonia in aqueous solutions. For instance, Cutolo et al. used electrospray pyrolysis to prepare a SnO 2 filmbased optical sensor for detecting low concentrations of ammonia in water [9]. Reflectance measurements were also applied for ammonia sensing using a colorimetric probe prepared by immobilizing Co(II) ions onto high-capacity Dowex HCR-W2 microspheres [10]. In this optode, the immobilized Co(II) ions form a blue hexaminecobaltate(II) complex with ammonia at pH 13. Another example is the optical sensor proposed by Salih et al. that used core-shell hyperbranched chitosan-based nanoparticles and gave a limit of detection (LOD) of 8 µg/mL [11]. In addition, Luo et al. used liquid crystal technology to design a chitosan/Cu 2+ -based optical sensor for ammonia determination, whose LOD was found to be 16.6 ppm [12]. An interesting flexible toolbox optical ammonia sensor relying on fluorescent aza-BODIPY dyes was developed by Mayr et al. [13]. The sensor measurements were carried out using a traditional phase fluorimeter combined with optical fibers, and they showed a very low LOD. All the above-mentioned assays are either very sophisticated, thus requiring expensive equipment, or having high LOD values. Therefore, an analytical method for detecting or quantifying very low concentrations of aqueous ammonia is highly needed. Furthermore, Amirjani and Fatmehsari developed a calorimetric method for the assay of ammonia based on AgNPs using smartphones. The change in color intensity was measured by a UV-Vis spectrophotometer, and the variation of the color intensity was monitored by RGB analysis using a smartphone's camera. Ammonia solutions within the concentration range of 10-10 3 mg L −1 were measured, while the detection limit was 180 and 200 mg L −1 [14].
In the present study, we aim at using PEI and PAMAM as capping agents for SPIONs that will allow for the in situ synthesis of AgNPs via chemical reduction due to the presence of free amino groups on the surface of PEI/PAMAMcoated SPIONs. We hypothesize that the presence of AgNPs in an ammonia-containing aqueous solution should affect the intensity of the SPR band [8]. Accordingly, the suggested platform can be used for quantifying ammonia in aqueous solutions using visible spectrophotometry. To the best of our knowledge, this is the first ammonia-sensing probe based on PAMAM-coated SPIONs decorated with silver nanoparticles.

Preparation of PEI-Coated SPIONs
SPIONs capped with PEI were prepared according to the method described elsewhere [15][16][17]. Briefly, 1 g of FeCl 3 .6H 2 O, 3 g of anhydrous sodium acetate, and 2 g of hyperbranched PEI were dissolved in 50 mL of ethylene glycol solvent with vigorous magnetic stirring for 2 h until the solution became homogenous. The prepared solution was then moved to a tightly closed Teflon-lined thermal autoclave (250 mL) and left in the oven at 200 °C for 6 h. The black-coated SPIONs were then collected by a strong magnet, washed three times with hot water and ethanol for the removal of any excess unreacted impurities, and then dried and stored for the subsequent experiments.

Preparation of Dendritic PAMAM
PAMAM hyperbranched polymer was prepared as described in the work reported by Salih et al. [11]. Briefly, the first generation (G1) of the hyperbranched polymer was obtained by the direct dropping of EDA solution (1 g/20 mL methanol) onto MA solution (7 g/20 mL methanol) in an ice bath (to slow down the reaction kinetics and avoid any agglomeration of the product). The reaction mixture was then stirred at room temperature for 48 h, and then all the excess unreacted precursors were removed by evaporation using a rotary evaporator. For building up the second generation (G2), an amount of the previously prepared solution (1 g/5 mL methanol) was added drop by drop to the EDA solution (0.6 g/10 mL methanol) in an ice bath under vigorous stirring, left to stir for 3 days at room temperature, and then purified by evaporation. In order to add terminal carboxylic acid groups to G2, an amount of the latter solution (1 g/5 mL methanol) was dropped onto MA solution (1.2 g/15 mL methanol) in an ice bath under vigorous stirring and then left to stir for 3 days at room temperature, followed by purification by evaporation.

Preparation of Dendritic SPIONs-PEI-PAMAM Nanoparticles
An amount of 0.2 g of SPIONs-PEI nanoparticles was treated with 1 g of hyperbranched PAMAM. Firstly, SPIONs-PEI were dispersed by sonication in an aqueous solution for 30 min. Afterwards, PAMAM was added, and the mixture was stirred for 5 days. The final coated SPIONs-PEI-PAMAM nanoparticles were then collected by a magnet and washed three times with hot water and ethanol. Finally, an aminolysis step was carried out for converting the remaining methyl ester groups of the synthesized SPIONs-PEI-PAMAM nanoparticles to terminal amino groups via soaking in an ammonia solution (10 mL NH 4 OH in 30 mL methanol) and stirring at room temperature for 3 days.

Preparation of AgNPs-Decorated SPIONs-PEI-PAMAM
To compare the sensitivity of the bare AgNPs with that of AgNPs-decorated SPIONs-PEI nanoparticles and AgNPs-decorated SPIONs-PEI-PAMAM nanoparticles, three samples were prepared by chemical precipitation. The three samples are as follows: (i) Naked Silver NPs (NP-I), (ii) SPIONs-PEI-AgNPs (NP-II), and (iii) SPIONs-PEI-PAMAM-AgNPs (NP-III). The concentration of silver (10 mL of 1 mM AgNO 3 solution) was kept constant, and a constant concentration of sodium borohydride (10 mL of 2 mM NaBH 4 solution) was used in the preparation of the above-mentioned three samples. In the case of NP-II and NP-III, 1 mg/mL of SPIONs-PEI and SPIONs-PEI-PAMAM were added to the AgNO 3 solution, respectively, and then the mixture was sonicated for 10 min. Thereafter, the NaBH 4 solution was dropped onto the resulting mixture at room temperature. The produced particles were collected by an external magnetic field, washed three times with hot water and ethanol, and then dried in the freeze dryer to obtain a powder. Figures 1 and 2 depict the scheme followed to prepare NP-III.

Calibration Curve Procedure
A calibration curve was plotted using a fixed weight of the sensor nanomaterial and different aliquots of ammonia solution while keeping the total volume constant. Practically, a 33% ammonia stock solution was used to prepare 10, 20, 30, 40, and 50 ppm solutions by dilution with double-distilled water. A fixed amount of each NP-III (1 mg/mL) was added separately to each of the mentioned concentrations and stirred for 2 h before UV-Vis measurements. Moreover, authentic samples of different ammonia concentrations were analyzed using the proposed nanosensor, and the recovery values were calculated. The precision of the suggested analytical method was evaluated by measuring the authentic samples three times and then calculating the relative standard deviation (RSD) of the obtained results. Three replicates of the standard curve were plotted, and the limits of detection (LOD) and quantification (LOQ) of the proposed analytical method were calculated as follows (Eqs. (1) and (2)) [18]: where σ is the standard deviation of the intercept of the standard plot, and S is its slope.

Results and Discussion
In the present work, the optical ammonia probe was synthesized from SPIONs using PEI as a capping agent. Thereafter, the capped SPIONs were hyperbranched and subjected to an aminolysis step, and then silver nanoparticles were deposited onto the terminal amino groups via in situ chemical reduction of silver ions into metallic silver. The following are the results obtained from the characterization and the analytical evaluation experiments carried out on the proposed sensor materials.

Electron Microscopy and EDX Analysis
The structure of NP-II is illustrated in the transmission electron micrographs shown in Fig. 3A-D. NP-II demonstrates a small and uniform size, thus revealing the efficiency of the applied solvothermal synthesis method. It is obvious from Fig. 3 that some regions contain PEI masses in close proximity to the PEI-coated magnetite nanoparticles. Upon enlarging the imaged nanoparticles (Fig. 3D), it can be clearly observed that every single nanoparticle is surrounded by a thin flare (i.e., a bright area) representing the coating PEI-AgNPs layer. On the other hand, NP-III appeared slightly bigger in size than NP-II (Fig. 3F). Meanwhile, Fig. 3G shows that the AgNPs were clearly noticeable on the surface of NP-III compared to its NP-II counterpart due to the presence of more available terminal amino groups, which reduce Ag + to Ag 0 . These AgNPs can also be seen very clearly in the enlarged inset of Fig. 3H. This finding elucidates the presence of a hyperbranched PAMAM layer on the surface of NP-III. Moreover, the selected area electron diffraction (SAED) of NP-II and NP-III shown in Fig. 3E and J appears as a ring pattern, which indicates that the synthesized nanomaterials are polycrystalline [19]. In addition, the presence of single spots in the SAED pattern of NP-II and NP-III may imply that the sample material is at the nanoscale [19].
SEM imaging was also used to investigate the presence of AgNPs on the surface of the hyperbranched PEI-coated magnetite nanoparticles (Fig. 4). The SEM images of NP-II ( Fig. 4A-C) depict that the sample contains two different types of nanoparticles. First, the smooth-edged, relatively large polygonal particles represent magnetite crystals coated with hyperbranched PEI. These particles appear heterogeneous in size and shape. Second, the smaller nanoparticles, which are more-or-less spherical in shape, represent the AgNPs deposited onto the PEI coating layer. On the other hand, although SEM images of NP-III (Fig. 4D-F) demonstrate the same types of particles observed in the SEM images of NP-II, the amount of AgNPs (i.e., the tiny spherical nanoparticles on the surface) is extremely larger in NP-III compared to NP-II. This observation may be due to the presence of a larger number of available reduction sites (i.e., terminal amino groups) on the surface of NP-III owing to the presence of the hyperbranched PAMAM.
EDX analysis was used to characterize the elemental composition of the synthesized nanocomposites. EDX analysis revealed the presence of oxygen, iron, and silver ions on the surfaces of both NP-II and NP-III (Fig. 4). The presence of AgNPs on the surface was confirmed by the strong sharp peak obtained at 3.0 keV [20] in the EDX patterns of NP-II ( Fig. 5A-C) and NP-III (Fig. 5D-F). The increase in the number of branches in PAMAM-coated nanoparticles (NP-III) results in a corresponding increase in the number of surface AgNPs, as indicated by the red dots in Fig. 5F. Therefore, the surface density of AgNPs can be tuned by changing the

Spectroscopic and Magnetic Properties
FTIR Spectroscopy In addition to the above-mentioned characterization techniques, the proposed nanocomposites were characterized with FTIR, XRD, and VSM. FTIR spectra were recorded over the spectral window of 4000-400 cm −1 by measuring the infrared transmittance of the samples via attenuated total reflection (ATR)-FTIR on a diamond crystal. The FTIR spectrum of NP-II (Fig. 6A) demonstrates two broad absorption bands at 3274 and 3132 cm −1 which correspond to -NH 2 stretching. These bands appeared broad because of the limited number of free terminal primary amino groups on the surface of NP-II as a result of the reduction of Ag + . Another absorption band appeared at 1635 cm −1 corresponding to amide C = O stretching. Meanwhile, the bands observed at 1365 and 1345 cm −1 can be attributed to C-H bending and the unreacted terminal CH 3 groups, respectively [11].
The FTIR spectrum of NP-III (Fig. 6A) is more or less similar to that of NP-II except that a sharp absorption band was observed at 1549 cm −1 in the spectrum of NP-III that was not observed in the spectrum of NP-II. This band can be assigned to the amide N-H stretching vibration, which indicates the presence of a larger number of amide groups (due to the presence of the hyperbranched PAMAM) in NP-III compared to NP-II (which contains PEI as a coating polymer) [11].
XRD Analysis X-ray diffraction analysis was carried out to characterize the prepared crystalline Fe 3 O 4 NPs, as well as to confirm the presence of AgNPs on the surface of the prepared nanomaterials (Fig. 6B) [19]. The same diffraction peaks could be observed in the diffractogram of NP-III, although some peaks were slightly shifted, minimized, or completely diminished. In addition, new peaks arose due to the deposition of AgNPs. For instance, the peaks perceived at the diffraction angles 37.7°, 43.8°, 62.3°, and 77.7° correspond to the diffraction  [21]. Unlike the diffractogram of plain SPIONs, which is characterized by strong, sharp diffraction peaks, the diffractogram of NP-III demonstrates less sharp peaks, implying that the lower crystallinity of NP-III is due to the presence of a thick capping layer of hyperbranched PAMAM.

Magnetic Susceptibility
In order to study the magnetic properties of SPIONs, NP-II, and NP-III, VSM analysis was used, where a magnetic field between − 20 and 20 kG was applied at 25 ± 2 °C (Fig. 6C). SPIONs showed superparamagnetic properties revealed by the high magnetization saturation (ca. 75.03 emu/g) and the low remnant magnetization and coercivity values [19]. In fact, these magnetic properties are excellent relative to those demonstrated by other magnetic nanomaterials reported in the literature. This may be due to the very small size of the prepared SPIONs. Upon coating with PEI-AgNPs, the magnetization saturation of the nanoparticles was decreased to 14.58 emu/g. A further decrease in magnetization saturation (reaching ca. 10.76 emu/g) occurred when the nanoparticles were coated with PEI-PAMAM-AgNPs. This indicates that the degree of hyperbranching is higher in NP-III compared to NP-II. Interestingly, this decrease in magnetic properties did not hinder the collection of the prepared nanoparticles by an external magnet. It is worth mentioning that the plain SPIONs did not show any response or sensitivity towards ammonia when treated with a 50-ppm ammonia solution (Fig. 7). It is also important to report that, despite the presence of particulate SPIONs in the measured samples, the measured absorbance signals were not affected because their particle size is very small, i.e., the nanoparticle solution usually has a clear yellowish color without any visible deposits. Therefore, by repeating the experiments, errors from light scattering were found to be at a minimum.

Analytical Applications
The electronic absorption spectra of a series of ammonia solutions of increasing concentration are illustrated in Fig. 8A-C. From the depicted results, it can be observed that increasing the ammonia concentration leads to a corresponding decrease in the measured absorbance values of NP-I, NP-II, and NP-III solutions. This result can be attributed to the coordination of ammonia molecules with the surface silver nanoparticles. AgNPs combine with ammonia molecules and generate a coordination complex Ag(NH 3 ) 2 + . Consequently, the formed coordination complex increases the surface positive charge of the AgNPs, thus increasing the repulsion force between the nanoparticles in the suspension. As a result, decolorization of NP-III takes place because the formation of Ag(NH 3 ) 2 + breaks away AgNPs from NP-III leading to diminishing of the silver plasmonic band in the visible light-absorption spectrum [22,23]. In other words, diminishing the SPR band of the nanoparticle system is attributed to the oxidation of AgNPs by ammonia (Ag 0 → Ag + ), which in turn decreases the SPR band of the decorating AgNPs [24,25]. Moreover, Fig. 8C shows that upon increasing the ammonia concentration in the suspension containing NP-III, a slight red shift is observed in the solution λ max . This shift may result in an experimental error in the optical measurements. Actually, this is not an issue because the absorption band is broad, and fixing the wavelength of measurement will not significantly affect the obtained results. This was confirmed by the very low RSD% values calculated after repeating the experiments. It can be clearly observed from Fig. 8A-C that NP-III is the most sensitive to changes in ammonia concentration since it demonstrates the highest change in absorbance intensity while maintaining the spectral pattern.

Linearity and Range
The standard/calibration curves were plotted for NP-I, NP-II, and NP-III (Fig. 8). The linearity of an analytical method is defined as the possibility of obtaining analytical signals directly proportional to the activity of the analyte in the sample [26]. Linearity can be evaluated by the calculated correlation coefficient (r 2 ) of the standard plot. Table 1 shows the correlation coefficient values of the analytical methods relying on NP-I, NP-II, and NP-III. It was found that the r 2 values of NP-II and NP-III are comparable (0.9910 and 0.9980, respectively); however, that of NP-I (r 2 = 0.9221) is significantly lower. Therefore, the linearity of the AgNPsbased method is not suitable for application in the assay of ammonia in aqueous solutions. On the other hand, the analytical methods that use NP-II and NP-III could be used successfully for the analysis of ammonia due to their large r 2 values.
One of the most important validation parameters of analytical methods is the applicability range, which can be defined as the interval between the lower and upper concentrations or amounts of the analyte where the analytical procedure has an acceptable level of accuracy, precision, and linearity [26]. The wider the applicability range, the better and more valid the analytical procedure. By applying the NP-I, NP-II, and NP-III systems, the standard curves were rectilinear over the specified concentration range of 10-50 mg/L. This indicates that the selected method (i.e., using NP-III) can be applied over a wide range of ammonia concentrations, thus minimizing the time required for sample pre-concentration or dilution.

Recovery Studies
One of the very important validation procedures, recovery studies, was carried out in order to evaluate the accuracy and precision of the suggested analytical method. Different known amounts of ammonia (5.0-45 µg) were taken and analyzed according to the above-mentioned standard plot procedures (using NP-III), and the found concentration was used to calculate the percent recovery values ( Table 1). The calculated recovery values (96.3-104.7%) were found to be in the acceptable range, thus indicating the high accuracy of the proposed analytical method. In addition, low relative standard-deviation values (RSD in the range of 2.4-4.2%) calculated from three replicate measurements were obtained, indicating the precision of the suggested analytical method.

LOD and LOQ
The LOD is an analytical validation parameter that evaluates the sensitivity of new analytical methods. It is defined as the lowest concentration of an analyte in a sample that can be consistently detected with a stated probability (typically at 95% certainty) [27]. The lower the LOD of an analytical method, the higher is its sensitivity. The LOD of the ammonia optical probe proposed in the present work is 5.69 mg/L, which is lower than all the previously reported optical ammonia sensors [14]. On the other hand, the LOD of NP-I (27.97 mg/L) and NP-II (15.33 mg/L) was significantly higher than that of NP-III, indicating that NP-I and NP-II are less suitable to be used for the analysis of ammonia in aqueous samples. This indicates that the proposed method has the advantage of having a higher sensitivity to ammonia compared to its peers. Moreover, the LOQ is the lowest concentration of the analyte that can be quantitatively determined with acceptable precision and accuracy [28]. The LOQ was found to be 93.24, 51.11, and 18.96 mg/L for NP-I, NP-II, and NP-III, respectively. All the measured and calculated analytical parameters indicate with out doubt that the NP-III probe is the most suitable and applicable sensor for ammonia determination in a water sample among the three studied systems.

Conclusions
In the present study, a newly developed optical sensor for ammonia detection in water samples is reported. Basically, the probe is formed of hyperbranched dendritic magnetic nanoparticles with a Fe 3 O 4 core and AgNPs-decorated PAMAM shell. The structure of the designed optical sensor nanomaterial was confirmed with different characterization techniques, and its ability to determine ammonia concentration in water was investigated. The sensor was successful in assaying low concentrations of ammonia in spiked water samples with high recovery and low standard deviation values, indicating its accuracy and precision. In addition, the sensor has the advantages of high sensitivity, high accuracy, very low concentration of the nanomaterial (i.e., inexpensive analysis cost), good linearity, and a wide applicability range. Although the sensor suffers from some probable sources of practical error, such as light scattering by the nanoparticles and the slight red shift observed in the electronic absorption spectra, these issues did not dramatically affect the analytical results. Currently, we are trying to enhance the performance of the proposed sensor by converting it to a fluorimetric probe in order to avoid these two limitations.