INTRODUCTION 
Organic reagents are chemical compounds that have been used for both quantitative and qualitative analysis of elements resulting in the formation of the colored complex [1-4]. Due to this reason, these reagents are widely employed for determining even the minute concentrations of several metal ions in different kinds of the materials [1, 5, 6]. The azo compounds-based complexes were reported to play a vital role in the biological fields because they have atoms like oxygen, nitrogen and sulfur, that allow them to bind with various metal ions [7, 8]. When comes to the selection of the metal, silver (Ag) is little harder than gold (Au). It is very ductile and malleable when compared with Au and palladium (Pd). Additionally, the electrical and thermal conductivity of the Ag metal is superior to all other metals while having the smallest contact resistance [9-11]. Ag is mainly sourced out from the copper ores [12] and finds wide range of applications in dental compounds, mirrors, electrical contacts, batteries, photography, jewelry, printed circuit, electronic industries [13-16] and for preventing bacterial development or growth in a variety of commercial products, like textiles [17]. Ag, at higher concentration, usually upto 2g of soluble Ag salts (in form of AgNO3), proved to be deadly. Ag compounds get easily absorb in the body tissues resulting in the bluish or blackish skin pigmentation [18-20]. Various methods including high performance liquid chromatography (HPLC) [21], kinetic [22-25], flame atomic absorption [26], solid phase extraction (SPE) [27], fluorimetric [28], electric method [29] and the spectrophotometric method [30-35] have been developed nowadays that determine the presence of the Ag ion. Among these methods, the spectrophotometric method was used in this study. Keeping in view the above discussion, the present study aimed to (i) synthesize new organic reagent (HMNTDABA), (ii) characterize the newly synthesized reagent by different techniques including FT-IR, UV-Vis, 1H-NMR spectra, XRD and FESEM techniques for determining the extent of this reagent’s interaction with 16 elements of the periodic table (iii) explore the interaction of this reagent with the Ag ion and (iv) determine the levels of Ag in some materials including dental filling. 

MATERIALS AND METHODS 
Instruments
Functional group study of the synthesized nano reagent was carried out by using Shimadzu Fourier transform infrared spectrophotometer (FTIR-8400S, Japan) within the spectral range of 400-4000 cm-1 in KBr disks [36-40]. The UV-Vis absorption spectra were measured by using Shimadzu UV-1800 UV/Visible scanning spectrophotometer while the absorbance was measured using 1 cm quartz cells. Further, MIRA3 device was used for the Field emission scanning electron microscopy (FESEM) analysis of the prepared materials. The CHNS analyzer, BBOT Model, was used to determine the carbon, hydrogen, nitrogen, and sulfur content in chemical samples. The X-ray diffraction (XRD) pattern of the prepared materials, a device (D2 Phaser) at an angular range of 2θ =10-80° was used. The pH of each solution was measured by using the pH meter (pH-3110 WTW, Germany) while the melting point measurements were carried out by using SMP30 Stuart, UK. 

Reagents and solution preparation 
All the chemicals used in the study were of analytical grade, and used without any further purification. Ethanol was purchased from Germany. The standard solution of organic reagent (HMNTDABA) having concentration of 1x10-4 M was prepared by dissolving 0.040g in 250 mL of absolute ethanol. For the preparation of the stock solution of Ag(I) having concentration of 1000 µg ml-1, nearly 0.157g of AgNO3 (Merck) was dissolved in 100 mL of deionized water. Other Ag (I) solutions were prepared by diluting the stock solution with distilled water. The pH of the solutions (3.4-10.5) was adjusted with ammonium acetate (0.01 mol L–1) ammonia-glacial acetic acid buffer solution. 

General procedure 
Firstly, the volumetric bottle with capacity of 10mL was taken to which 2mL of silver ion (I) solution having concentration of 100 ppm was added. This was followed by the addition of 3mL HMNTDABA nano reagent (Fig. 1) solution with a concentration of 1x10–4 M. The total volume of the solution was made up to the mark with distilled water. The final concentration of the obtained complex was 100 ppm having pH solution of 8. The absorbance of the resulting solution was then measured after 5 min at the λmax of 563nm at the temperature of 25°C against the nano reagent solution prepared in the same way in the reference cell (without silver solution). 

RESULTS AND DISCUSSION
FTIR and FESEM analysis of the newly synthesized nano reagent (HMNTDABA) and its complex 
The functional group study of the synthesized nano reagent and its complex was carried out within the range 400–4000 cm-1 [41-44]. The results of the FTIR study for the synthesized reagent and its complex are elaborated in Fig. 2. The bands of the azo reagent at 3425.3 cm-1 and 3240.2 cm-1 corresponds to the presence of the O-H bond of the carboxylic acid and the alcoholic O-H bond, respectively. The peaks obtained within the range 3016.46 cm-1 to 3062.75 cm-1 corresponds to the aromatic C-H bond vibrations while the peaks ranging from 1442.66 cm-1 to 1550 cm-1 corresponds to the N=O and N=N bond vibrations of the nitro group and the azo group respectively. Furthermore, the peaks within the range 1660 cm-1 to 1658.67 cm-1 corresponds to the presence of the C=C and C=O vibrations. The functional group study of the silver complex revealed the change/ shift in the apparent peaks of the nano reagent. The peaks obtained within the range 3200 cm-1 to 3550 cm-1 revealed the presence of the O-H group in the complex while the bonds appeared within the range 470 cm-1 to 672 cm-1 showed the presence of the Ag-N and Ag-O bond vibrations respectively. The FESEM images of the nano reagent in Fig. 3 reveal the morphological characteristics of the studied compound and its microstructural characteristics. The surface of the nano reagent is heterogeneous possessing numerous irregularities. Particle sizes, however, appears to be consistent across the sample highlighting the controlled synthesis process. Additionally, the presence of the clusters in images also highlights the aggregation behavior of the particles that would affect the properties of the materials. The FESEM images of the prepared complex reveal the agglomeration of Ag particles on the surface of the nano reagent. This observation supports the coordination between the nano reagent and the Ag particles Fig. 3 [45-48].  

Elemental analysis 
Elemental analysis of a compound helps in determining the type of elements and their respective percentages in compound. The results of the elemental analysis (as summarized in Table 1) show that the studied compound is chiefly composed of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) in percentages of 46.999%, 2.723%, 17.139% and 7.677% respectively. Furthermore, the calculated molecular weight of compound i.e., 401.364 g/mol is consistent with the molecular formula i.e., C16H11SN5O6. 

1H-NMR spectrum and XRD analysis for organic reagent 
The synthesized nano reagent i.e., HMNTDABA in DMSO-d6 solvent was analyzed by using 1H-NMR spectrum and the results are shown in Fig. 4. The results elaborated in the Fig. 4 show the signal appear at a location of 52 ppm. The 1H-NMR spectrum of HMNTDABA showed the band at a frequency of 98 ppm representing the presence of the OCH3 group. Additionally, the bands appeared at 97ppm and 45ppm represents the proton number 15. The location of 1.4 ppm belongs to the OH proton while the presence of the proton of the thiazole ring take place at a frequency of 57.8 ppm. The XRD analysis of the prepared nano reagent, HMNTDABA, has been outlined in Fig. 5. The results of the study highlight the complex crystalline structure of the HMNTDABA having numerous diffraction peaks at various 2θ positions, notably at 11.5744°, 31.8404°, and 38.3375°, with the highest intensity observed at 38.3375°. The results of the XRD provide important information regarding the crystallinity and structural properties of the newly synthesized nano reagent [49-51]. 

Absorption spectra and characteristics of the nano reagent and relativecomplex
Fig. 6 revealed the absorption spectrum of the silver complex (I) with the nano reagent, obtained within the wavelength range of 200-800 nm. Results of the study revealed the presence of the red shift in the spectrum due to the electronic transition i.e., π→π* at the λmax of 563 nm. As compared to the Planck solution as a reference, the solution with pH=8 was at the highest absorption peak. Furthermore, when the absorption spectrum of the nano reagent was compared with the absorption spectrum of the complex, a clear difference in the values of λmax for both the nano reagent and the complex was observed. Additionally, the red shift occurred towards the longer wavelength in the spectra. These results are the evidence of the occurrence of a coordination process taking place between the metal ion and the nano reagent during the synthesis of the complex[38]. 

Parameters 
Effect of pH
The results of the pH study in Fig. 7 reveal the effect of acid function values on the absorbency of the silver (I) complex. It was observed that the color intensity of the complex solution increases gradually reaching its peak at the acid level of 8 afterwards a decrease in the absorbance value of the complex take place. This decrease might be due to the deposition of the element ion or due to the formation of unstable complex ions [49].   

Effect of the temperature and time
The effect of the temperature on the absorption of a silver complex solution has been shown in Fig. 8. Results of the study revealed that the absorption values of the silver complex solution give the best color intensity at temperature range of 10-40 °C afterwards it starts decreasing with further increase in temperature. This effect can be better explained by the disintegration of the complex at the high temperatures or the conversion of some part of the complex into another form when the solution temperatures exceeds 40 °C [52]. 
The effect of the time on the stability of the absorbance of the silver complex (I) solution is shown in Fig. 9. Results of the study revealed that after the time of 5 min, the absorbance of the prepared complex remains almost constant till time period of 24 h. The results of the study showed that the studied complex has a high stability and 5 min is the optimum time to complete the reaction between silver ion (I) solution and the nano reagent solution.

Complex composition and stability constant 
For the determination of the composition and stability constant, both the continuous variation (Fig. 10) and the mole ratio approaches (Fig. 11) were used. It was observed that at pH = 8, the molar ratio of silver (I) ions to the nano organic reagent is 1:1 (metal: reagent) that was also confirmed by both approaches (continuous variation and the mole ratio). Following equation helps in determining the stability constant (Kst =0.5x10-6 ) and the complex  as follows: 

where α represents degree of dissociation, c refers to the total concentration of the complex, n denotes mole ratio i.e., (1 to Ag(I)), Am refers to the absorbance of a solution containing nano reagent two times higher than the silver amount and As refers to the absorbance of a solution containing a stoichiometric amounts of both [reagent] and [silver]. 

Characteristics of analysis
 According to the experimental protocol, the calibration curve was made, and a good correlation coefficient was observed from the results (Fig. 12). The analytical parameters for this spectrophotometric analysis of Ag (l) with the synthesized nano reagent are summarized in Table 2.  

Silver (I) complex investigation by interference  
The divalent cations (Fe+2, Ni+2, Zn+2, Pb+2, Cu+2, Cd+2) formed complexes with the synthesized nano reagent, HMNTDABA, during its reaction with silver. The effects of these ions were examined and the results are summarized in Table 3.

Determination of silver (I) in dental fillings  
The procedure involved the dissolution of 0.1 g of dental filling in 10 mL of concentrated nitric acid. The resulting solution was filtered, and the filtrate was transferred to an opaque 100 mL volumetric flask, which was then diluted to the mark with distilled water for Ag measurement. Subsequently, half of the resulting solution was placed in a 50 mL volumetric flask, and the pH was adjusted to 8 using buffer solutions. Blocking agents were then added to inhibit the copper present in the solution. Finally, under optimal conditions, the silver ion (I) concentration was determined using the prepared nano reagent, HMNTDABA, via the spectroscopic method. 
Followed by the application of the working method in a dental filling model (Australia by SDI Limited, Fig. 13), the proportions of its components were determined including Ag = 40.0%, Cu = 28.7% and Sn = 31.3%. The results of the study revealed that the percentage of Ag ion was determined to be 39.44 % with the relative recovery of 98.6%. The results revealed that the studied method has good sensitivity and can be employed for the investigation of the Ag ions [53].

CONCLUSION 
This study involved the synthesis of the novel nano azo reagent i.e., HMNTDABA for the preparation of the Ag (I)-complex. Successful synthesis of the nano reagent and its complex was investigated by the use of different characterization procedures including UV-Vis spectroscopy, infrared spectra, 1H-NMR spectroscopy, XRD, FESEM and CHNS. Effect of pH, contact time, and temperature on complex formation was also investigated and results of the study revealed the pH of 8, time of 5 min and 25 °C as the optimum temperature. The applicability of the prepared silver complex was checked on a dental filling model to estimate the Ag ion using the spectroscopic method. The results revealed that the studied method has good sensitivity and can be used to estimate the silver ion. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.