Introduction

With the importance of nanomaterials in all fields of science, their diverse application, and the rapid development of the nanotechnology industry, the research activities of many groups are aimed at synthesizing nanomaterials with specific and desired properties (Ali et al. 2018a, 2018b, 2019a, 2019b, 2021; Khan et al. 2020). Lately, numerous works have been reported for the preparation of metals, metal chlorides, metal sulfides, and metal oxides by the various procedures. Among inorganic nanoparticles (NPs), silver-based nanomaterials, such as silver (Ag), silver chloride (AgCl), and Ag/AgCl nanoparticles have been extensively applied due to their wide physical, chemical, photochemical, biological, biomedical, and antimicrobial properties (Gopinath et al. 2013; Nikaeen et al. 2020; Stalin Dhas et al. 2014; Villanueva-Ibáñez et al. 2015). For these specific properties, Ag-based nanoparticles have gained very application in wastewater treatment, photocatalytic activity and degradation of dyes and organic pollutants, antifungal, antibacterial, antibiofilm, antioxidant, and anticancer agent, and chemical sensing (Devi and Ahmaruzzaman 2016; Feizi et al. 2018; Han et al. 2018; Lombi et al. 2013; Narayanan and Han 2017). Silver-based nanomaterials have been produced by chemical, physical, or biological routes. The physical methods are generally energy-consuming and require sophisticated instruments. The chemical processes involve the utilization of reducer and stabilizer, toxic solvents and chemicals, and generating hazardous waste, that cause restriction of their application in the clinical and biomedical areas. Hence, there is a growing request for cheaper, simpler, cleaner, nontoxic, and ecologically friendly protocols that tend to be available and renewable resources (Eugenio et al. 2016; Gopinath et al. 2013; Kulkarni and Bhanage 2014; Samari et al. 2019; Stalin Dhas et al. 2014). The biological synthesis using various biological systems such as plants, fungi, bacteria, yeasts, actinomycetes, and viruses displays a suitable substitute to physicochemical methods in the synthesis of silver-based nanomaterials. The phyto-synthesis of nanoparticles has a profitable approach and gained significant attention because it is rapid, simple, cost-effective, and easily scaled up to produce well-dispersed nanoparticles with uniform size and shape (Kulkarni and Bhanage 2014; Sharmila et al. 2018; Villanueva-Ibáñez et al. 2015). There have been many reports on the successful production of silver-based nanoparticles, especially Ag NPs, by plant-mediated extracts (Azarudeen et al. 2017; Manivel and Ilanchelian 2017; Patil Shriniwas 2017; Rajan et al. 2015; Rautela et al. 2019; Samari et al. 2018). In contrast, reports on the phyto-synthesis of Ag/AgCl NPs are less frequent (Devi et al. 2016b, 2016a; Kulkarni and Bhanage 2014). On the other hand, in very fewer cases, the effect of different parameters on the synthesis of Ag/AgCl NPs by plant extracts has been investigated (Al Aboody 2019). Thanks to the existence of various plants and their potency to synthesize nanoparticles, there are several occasions to obtain technologically interesting materials (Villanueva-Ibáñez et al. 2015).

S. cumini is from the Myrtaceae family and is an evergreen tropical tree with possesses hypoglycemic, anti-inflammatory, and antioxidant properties (Bitencourt et al. 2017; Bitencourt et al. 2016). All parts of the S. cumini have been widely applied for different medicinal purposes such as antidiabetic, astringent, carminative, stomachic, anti-scorbutic, and diuretic (Mittal et al. 2014). Various phyto-compounds are presented in this plant such as phenols, flavones, tannins, glycosides, amino acids, and alkaloids (Prasad and Swamy 2013). From literature, polyphenols and flavonoids are present in high levels in S. cumini. These phyto-molecules are expected to act as reducing agents and cap the formed nanoparticles (Bajpai et al. 2005; Bitencourt et al. 2017; Prasad and Swamy 2013).

Clindamycin (CDM) (Additional file 1: Fig. S1), methyl 6-amino- 7-chloro-6,7,8-trideoxy-N-[(2S,4R)-1-methyl-4-propylprolyl]-1-thio-L-threo-D-galacto octopyranoside-2-(dihydrogen phosphate) is a well-known antibiotic with routine application to treat infections of the respiratory tract, skin, and soft tissue. The bacteriostatic action of clindamycin is primarily against gram-positive aerobes and can be also effective for the anti-infection of a wide range of anaerobic bacteria (Hadi and Honarmand 2017; Ibrahim et al. 2017). On the other hand, a trivalent form of iron (Fe3+) which is the most indispensable and abundant in the natural environment has decisive roles in biological systems, physiological or pathological events (Faizi et al. 2016; Li et al. 2017). Its deficiency or overdose can induce variant disorders (Qin et al. 2008). Additionally, to limit the side effects of drugs based on Fe2+ ions, such as gastrointestinal irritation and abdominal pain, the usage of Fe3+ complexes has been introduced (Abbaspour et al. 2006). Given the extraordinary importance of clindamycin and Fe3+ ions in biological, clinical, and environmental applications, it is really necessary to develop a new and direct determination method for these analytes. Several analytical methods can be found in the literature for the quantitation of clindamycin and iron ions such as electrochemical methods (Hadi and Honarmand 2017; Hu et al. 2016; Wong et al. 2016), spectrofluorimetric method (Faizi et al. 2016; Wahba et al. 2015; Wang et al. 2016), chromatographic methods (Pereira et al. 2018; Schnell et al. 1998), mass spectrometry (Aydin and Soylak 2010; Girondi et al. 2018), and atomic absorption spectroscopy (AAS) (Shamspur et al. 2005). Although each technique has its own merits in one aspect, most of these methods require the use of organic solvents and derivatization of the compound or selective detectors, sophisticated pretreatment, expensive apparatus, elaborate multistep extraction procedures, and trained staff (El-Yazbi and Blaih 1993; Faizi et al. 2016; Li et al. 2017; Vallejos et al. 2016). Therefore, there is still a need for a precise, easy, and convenient alternative method for the measurement of the intended analytes. Moreover, no straight spectrophotometric procedures exist to measure clindamycin. The simplicity and economic advantages of spectrophotometric methods make them highly preferable routes for routine analysis. The lack of an absorbing peak in the UV–Vis region is the main problem in the spectrophotometric determination of clindamycin. In the previous reports, a derivative reaction for the conversion of clindamycin into a UV–Vis active compound was used to measure its concentration spectrophotometrically (Barazandeh Tehrani et al. 2013; El-Yazbi and Blaih 1993), which is not generally an easy and reproducible procedure. So, to quantify the clindamycin in the pharmaceutical dosage form, the development of direct spectrophotometric methods is required (El-Yazbi and Blaih 1993). Considering the abovementioned difficulties, colorimetric sensors rendered an efficient tool for the determination of intended analytes. The colorimetric sensors have some advantages such as ease of construction, simplicity, inexpensive, rapid, and more reliability, and it is possible to have convenient data collection by various cameras or even using the naked eyes (Li et al. 2017).

This study focused on the phyto-synthesis of Ag/AgCl NPs from aqueous fruit extract of S. cumini. The effects of different empirical variables on the formation of nanoparticles, such as pH of the solution, concentrations of the silver solution, amount of fruit extract, temperature, and reaction time were studied. To the best of our knowledge, S. cumini fruit extract has not been reported for the synthesis of Ag/AgCl NPs despite a recent report on the synthesis of Ag NPs by fruit extracts of S. cumini (Mittal et al. 2014). The prepared Ag/AgCl NPs were analyzed by different common characterization tools. Furthermore, phyto-synthesized Ag/AgCl NPs using aqueous S. cumini fruit extract were evaluated for their potential as colorimetric sensors toward clindamycin and Fe3+ ions.

Materials and methods

Chemicals

Silver(I) nitrate (AgNO3), mercuric(II) chloride (HgCl2), and iron(III) nitrate (Fe(NO3)3) were purchased from Merck chemical company, Germany. Clindamycin phosphate was taken from the Sepidaj pharmaceutical company based in Iran. All chemicals used were of reagent grade and were used as received. The solutions of all metal salts (0.1 M) for our runs were prepared from their corresponding metal salts in deionized water. The cleaning of all glassware was intensively attended by rinsing several times with tap water and then re-washed with the deionized water. Deionized Millipore water was used for the reagent preparation. Diluted solutions of hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to set the pH wherever required.

Preparation of S. cumini fruit extract

S. cumini fruits were collected from Bandar Abbas city, Hormozgan province, Iran. The surfaces of freshly collected fruits were cleaned with running tap water, followed by deionized water. Then, the fruit is separated from the core and the prepared fruit samples were dried in shade for 8–10 days, and later, they were finely powdered and stored in a stoppered bottle at the refrigerator at 4 °C for further studies. To prepare fruit extract, 10.0 g of the dried fruit powder and 150 mL of deionized water were mixed in a 250-mL conical flask and then was heated at 80 °C for thirty minutes. After getting cooled, the filtration of the mixture was done through normal filter paper followed by Whatman No. 1 paper. The obtained extract was stored in a refrigerator for further assays and used within a week.

Synthesis of Ag/AgCl NPs

The method includes the simple addition of S. cumini fruit extract into the silver(I) nitrate solution at ambient conditions. For typical experiments, a 50-mL Erlenmeyer flask was utilized for mixing 2.0 mL of S. cumini fruit extract with 25 mL of 1.0 mM aqueous solution of silver(I) nitrate, and vigorously stirring was applied for 90 min. The reaction development for the production of Ag/AgCl NPs was confirmed from the changing of its color, which was appeared visually from watery yellow to dark brown, and has been followed up quantitatively using UV–Vis spectra.

The various parameters have an effect on the size, shape, dispersity, and optical characteristics of nanoparticles (Singh et al. 2017; Vanaja et al. 2013). Thus, to determine the ideal conditions, the pH of the mixture, the volume of extract, the quantity of metal precursor, temperature of the media, and incubation time were optimized.

The effect of the pH of the mixture solution (3.0, 5.0, 7.0, 9.0, and 11.0), amount of fruit extract (1.0, 2.0, 3.0, 5.0, 7.0, and 10.0 mL), concentration of silver precursor (0.5, 1.0, 2.0, 4.0, and 7.0 mM), the mixture temperature (room temperature (R. T.), 40, 60, and 80 °C), and reaction time on the synthesis of Ag/AgCl NPs were investigated; by keeping constant all parameters except one. The phyto-synthesized NPs were centrifuged at 10,000 rpm for 30 min and rinsed with deionized water. Re-dispersion of the resulting sediments was done in deionized water to rid of any free biological compounds. This procedure was replicated three times to ensure better separation of uncoordinated entities. The dried sediments were used for characterization processes such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy–energy-dispersive X-ray analysis (FESEM–EDS).

Characterization of the synthesized Ag/AgCl NPs

UV–Vis spectra were obtained by SCINCO-S-3100 (Korea) spectrophotometer equipped with a 1.0-cm quartz cell, and all experiments were done at room temperature. The FTIR spectrum was done in the range of 4000–500 cm−1 using Bruker alpha (Germany), and an attenuated total reflectance (ATR) cell was utilized for spectral recording. The assessment of the crystal structure of the prepared nanoparticles was performed using a Bruker D8 X-ray diffraction (XRD) (Germany) with Cu-Kα radiations. The identification of shape, size, and the elemental study of the prepared Ag/AgCl NPs was performed using sigma-VP (Zeiss, Germany) field emission scanning electron microscopy–energy-dispersive X-ray analysis (FESEM–EDS). Transmission electron microscopy (TEM) studies were carried out by Zeiss–EM10C with an accelerating voltage of 80 kV.

Colorimetric sensing of clindamycin using Ag/AgCl NPs

The Ag/AgCl NPs-based colorimetric method was used to measure clindamycin. The pH of the sensing solution and reaction time were optimized. To optimize the pH of the reaction mixture, the sensing process was run at different pH (3.0–11.0 with an interval of 2). That way, 35.0 µL of 0.01 M of clindamycin solution was injected into 2.5 mL of diluted solution of Ag/AgCl NPs (with ratio 1:4) with adjusted pH, and the alteration in the color of the obtained mixture was followed using UV–Vis absorption spectra. At the optimized pH, the absorbance of the reaction mixture was measured at different time intervals. In following, at the optimized reaction conditions, the calibration curve was obtained by consecutive injecting of different µL volumes of clindamycin into 2.5 mL of the diluted Ag/AgCl NPs.

To investigate the selectivity of the proposed sensor for clindamycin, different drugs (atenolol, antihistamine, cimetidine, glibenclamide, metformin, melatonin, gentamicin, ranitidine, erythromycin, clarithromycin, rifampin) were checked. Some of these drugs have similar use/activity, and some others have been patented because of their broad spectrum of activity (Paul et al. 2017; Pereira et al. 2018). Because of the popularity of clindamycin and some other list of antibiotics, there are falsified and fake copies of the antibiotics on the market (Paul et al. 2017) and the exact determination of clindamycin is important. Some of these selected drugs such as rifampin also are currently used in combination with clindamycin to treat severe infectious diseases in hair follicles (Pereira et al. 2018; van der Zee et al. 2009). On the other hand, some of the selected compounds were studied as interference to show the potential of determination of clindamycin in wastewater or complex biological samples for future works. Each of these drugs (50.0 µM) was added separately into 2.5 mL of diluted Ag/AgCl NPs solution at the optimized pH, and the sample vials were kept for a few minutes at room temperature. The changes of color were monitored via a digital camera and the UV–Vis absorption spectrometer.

Finally, the proposed colorimetric sensor was applied for determinations of clindamycin in pharmaceutical formulations. Two commercial pharmaceutical formulations of clindamycin (clindamycin capsules from Daroupakhsh distribution company, Iran, contain 150 mg clindamycin hydrochloride per capsule based on its commercial label, and clindamycin injection dosage, 150 mg mL−1, from Tehran Chemie Daroo, Iran) were purchased from local drugstores and used as real samples. Ten capsules were finely grounded and well mixed. The equivalent of one capsule was accurately weighed, dissolved in deionized water, and then filtered by 0.45-µm pore size Biofilm filters, to separate insoluble materials. After the transfer of the solution to a 10-mL volumetric flask, the final volume was adjusted to the mark with deionized water. The concentration of clindamycin was 0.035 M in this prepared solution. 8.6 µL of the above-prepared clindamycin capsule solution was added into 2.0 mL of the freshly synthesized Ag/AgCl NPs solution in 10-mL volumetric flasks, and the volume was adjusted with deionized water and mixed well. Determination of clindamycin was done via the standard addition approach. For injection dosage form of clindamycin, 0.86 µL of the ampoules, labeled to contain 150 mg mL−1 (equivalent to 0.35 M) of clindamycin phosphate per ampoule, was added into 2.0 mL of the freshly synthesized Ag/AgCl NPs in 10-mL volumetric flasks and the next steps were the same approaches as previously mentioned.

Detection ability of the synthesized Ag/AgCl NPs for Fe3+ ions

The detection capability of the as-synthesized Ag/AgCl NPs to metal ions was checked. For this goal, different metallic salts ions (Li+, Na+, K+, Ba2+, Ca2+, Mg2+, Al3+, Bi3+, Cd2+, Co2+, Cu2+, Fe2+, Fe3+, Hg2+, Mn2+, Ni2+, Sn2+, and Zn2+) at the same conditions (10.0 µL, 10−1 M) were added into the diluted solution (with ratio 1:4) of Ag/AgCl NPs (2.5 mL). UV–Vis absorption spectra were utilized to follow the alteration in the color of the examined solutions. After surveying the UV–Vis absorption spectra and color change of the assay solutions after the addition of different metal ions, the Fe3+ ion was selected for further sensing. In the following, the influence of the reaction mixture pH and time was optimized to get the highest sensitivity in the determination of Fe3+ ions.

In the following, the spectrometric detection of Fe3+ ion was carried out by adding different volumes (in microliter) of a solution of 0.001 M Fe3+ into the 2.5 mL of the diluted solution of Ag/AgCl NPs (adjusted to optimized pH). The absorption spectra of the mixture were monitored via a UV–Vis spectrophotometer.

The applicability of the suggested approach for the determination of Fe3+ in the real medium was checked using two water samples: tap water (Molecular Medicine Research Center, Hormozgan Health Institute, Hormozgan University of Medical Sciences, Bandar Abbas, Iran) and mineral water (MALAS Company, Iran). The concentration of Fe3+ in the studied real samples was initially checked by atomic absorption spectrometry (240 FS, Agilent Technologies, USA). In this work, 1.0 mL of each kind of water was tested in a 10-mL volumetric flask, mixed with 2.0 mL of the synthesized Ag/AgCl NPs, adjusted to the mark with deionized water, and mixed well. The standard addition method was used for quantitation of Fe3+ ions in such a way that into 2.0 mL of the above solution (adjusted to optimized pH) various volume of a standard solution of Fe3+ was added, and the absorbance of the obtained mixture was collected 1 min after each addition.

Results and discussion

Visual observations and UV–Vis spectral studies

The addition of the aqueous fruit extract of S. cumini to the prepared silver ions solution causes a color alternation in the mixture, and it changed from light yellow to brown. This color change of the mixture solution qualitatively confirmed the production of Ag/AgCl NPs and can be spectrally followed by the surface plasmon resonance (SPR) of the formed nanoparticles (Mulvaney 1996; Tripathy et al. 2010). Furthermore, an intense and sharp SPR peak at around 412 nm confirmed the formation of Ag/AgCl NPs as shown in Fig. 1. Although numerous methods are reported in the literature for Ag/AgCl NPs synthesis, most of them used toxic materials (Song et al. 2013; Wang et al. 2008). The use of green and clean technology is expected to minimize pollution in the environment. In this work, S. cumini fruit extract was adopted for the green fabrication of Ag/AgCl NPs because it is inexpensive and environmentally friendly. The available phytochemicals in the S. cumini fruit extract can serve multiple functions as reducing and stabilizing agents and halide sources in the synthesis of Ag/AgCl NPs. The proposed method is a novel approach that avoids the use of any external reducing and capping agents, toxic solvents, and external halide ion sources.

Fig. 1
figure 1

UV–Vis absorption spectra of the Ag/AgCl NPs synthesized at a different pH from 3 to 11, b different S. cumini fruit extract content from 1 to 10 mL, c different silver nitrate concentration from 0.5 to 7.0 mM, d different reaction temperature (R. T. , 40, 60, and 80 °C), and e optimization of the reaction time

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Optimization of the phyto-synthesis of nanoparticles

To study the effect of pH on the production of NPs, the synthesis procedure was run at different pH (3.0, 5.0, 7.0, 9.0, and 11.0), while, the other parameters like the quantity of silver precursor (25 mL, 1.0 mM), the volume of S. cumini fruit extract (2.0 mL), reaction temperature (R. T.), and incubation time (1.5 h) were kept constant. The UV–Vis spectra of the mixture at different pH were monitored during the production of Ag/AgCl NPs and are shown in Fig. 1a. Our results show that the pH of the reaction has a main effect on the construction of Ag/AgCl NPs. At pH of 3.0, no characteristic peak of Ag/AgCl NPs was observed, due to suppression of the nanoparticle production in acidic media (Vanaja et al. 2013). As the pH of the reaction media increased from 3.0 to 7.0, the SPR intensity of the formed nanoparticles gradually increased accompanied by the blueshift from 407 to 404 nm. At pH 9.0, the absorbance intensity was near to its value at pH 7.0, although the SPR band became wider and the λmax is shifted to 400 nm which indicates smaller particles synthesis with a wider size distribution. These observations suggest the formation of smaller particles with higher formation rates in media with basic and neutral pH that is due to the ionization of phenolic compounds in the utilized extract (Samari et al. 2018). The low production rate of nanoparticles in acidic media is due to the lower activity of phyto-molecules reducer and the aggregation of synthesized nanoparticles to form larger particles (Saware and Venkataraman 2014). At pH 11.0, the absorbance intensity was decreased and the peak broadening was observed. Also, agglomeration or precipitation was seen after 24 h at higher pHs (9.0 and 11.0) because of the lower stability of the synthesized nanoparticles or the absence of a stabilizer (Vanaja et al. 2013). It was found that a neutral medium (pH of 7.0) produced more stable nanoparticles with a smaller size distribution. Therefore, pH 7.0 was considered as an optimum pH for further study.

In Fig. 1b, the absorption spectra of the synthesized nanoparticles by adding the various quantities of fruit extract (1.0, 2.0, 3.0, 5.0, 7.0, and 10.0 mL) to 25 mL of 1.0 mM of AgNO3 were shown (the temperature, incubation time, and pH were R. T., 1.5 h, and 7.0, respectively). The resulting spectra have shown a great dependency of the formed nanoparticles on the quantity of fruit extract that acts as a capping and reducing agent. By increasing the quantity of extract from 1.0 to 5.0 mL, the SPR intensity was increased, mainly due to the construction of more nanoparticles. Also, the λmax of the SPR band shifted to a longer wavelength, which confirms the formation of larger particles. In fact, by increasing the volume of fruit extract, excess amounts of reducing agents in the extract could be bound to the preformed nuclei’s surface and cause the additional reduction of metal ions. Subsequently, the increase in the growth rate of nanoparticles happened and larger particles are formed (Tahir et al. 2015; Yang and Li 2013). Having a further increase in the extract volume (7.0 and 10.0 mL), a decrease in the absorbance accompanied by band broadening was observed, as presented in Fig. 1b, which indicates a larger size distribution of the formed nanoparticles. Concerning the stability of the synthesized nanoparticles, the application of a 3.0 mL fruit extract solution for reacting to AgNO3 solution causes more stability than others. It was reported that a high quantity of reducing agents can enhance the bridging effect between the formed nanoparticles and lead to the aggregation of nanoparticles (Yang and Li 2013). From the above discussion, the optimized quantity of S. cumini fruit extract for the preparation of Ag/AgCl NPs in the presence of 25 mL of AgNO3 (1.0 mM) was selected to be 3.0 mL.

The effect of concentration of AgNO3 solution during the preparation of nanoparticles was examined by exposing 3.0 mL of fruit extract of S. cumini to 25 mL of AgNO3 with different concentrations (0.5, 1.0, 2.0, 4.0 and 7.0 mM) at pH 7.0 and room temperature for 1.5 h. The UV–Vis spectra of the resultant Ag/AgCl NPs colloidal solution by different concentrations of the silver salt solution are shown in Fig. 1c. By enhancing the concentration of silver precursors from 0.5 to 4.0 mM, the SPR intensity was enhanced due to an increase in nanoparticles formation. Thereafter, at 7.0 mM, the absorbance intensity started to decrease because the applied quantity of fruit extract might not be enough to reduce all existing silver ions, and hence peak intensity was decreased. Also, with an increase in the silver ion concentration, the SPR band shifted to red (from 412 to 421 nm) and band broadening occurred, which represents the synthesis of larger particles with a wider size distribution. The reason for this observation is an enhancement in agglomeration after the addition of more metal ion amounts (Saware and Venkataraman 2014). It is necessary to mention that similar results have been obtained in other researches (Vanaja et al. 2013; Yang and Li 2013). Therefore, 1.0 mM of AgNO3 solution is more appropriate to get a high concentration of nanoparticles with a smaller size, more stable, and narrower size distribution.

To study the effect of reaction temperature on the synthesis of nanoparticles, the reaction was done at four different temperatures (room temperature, 40, 60, and 80 °C), keeping the volume of fruit extract constant at 3.0 mL, 25 mL of 1.0 mM of AgNO3, pH of 7.0, and 1.5 h. The resulting absorbance spectra of the synthesized nanoparticles at four temperatures are shown in Fig. 1d. Based on the obtained data, it could be concluded that the reduction process is related to the reaction temperature and a higher temperature leads to more production of nanoparticles. It is worthy to note that in higher temperature (> 80 °C) such as 90 °C, no enhancement was observed SPR peak. On the other hand, the SPR in high temperature (80 °C and higher) was not repeatable but repeatability was better in 60°. However, a more intense SPR peak was obtained at 80 °C, for energy saving and more difficulty in the adjustment and to obtain repeatable SPR, the temperature of 60 °C was utilized as the optimum.

The kinetics of the synthesis procedure was performed by following the reduction process and recording the absorbance value at different time intervals. Figure 1e shows the SPR intensity at 412 nm from the reaction between aqueous silver nitrate and fruit extract (at the optimum condition) as a function of time. It was found that the production of nanoparticles is started almost immediately after the addition of the fruit extract, and this continued throughout the investigation process (as shown by the progressive enhancement in the absorbance intensity at 412 nm) and remained unchanged after 40 min of incubation, that is confirmed the reaction process is completed at 40 min. Thus, it is obvious that the S. cumini fruit extract has a potent capability to make and stabilize nanoparticles and the reduction was done faster than some of the reports about the plant-mediated synthesis of Ag/AgCl NPs (Chen et al. 2000; Chyau et al. 2006; Kinoshita et al. 2007).

Characterization of nanoparticles

X-ray diffraction (XRD) studies

The crystalline structure of the Ag/AgCl nanoparticles was identified using XRD study, and the obtained pattern is shown in Fig. 2a. The peaks at 2θ values of 27.8°, 32.2°, 46.2°, 54.9°, 57.6°, 67.4°, 74.9° and 76.7° have belonged to the (111), (200), (220), (311), (222), (400), (311), and (420) lattice planes of the cubic silver chloride NPs, respectively (ICDD file no. 31-1238) (da Silva Ferreira et al. 2017; Zhao et al. 2015). The remaining peaks at 38.2°, 44.6°, 64.4° and 77.2° were indexed of the formation of (111), (200), (220), and (311) planes in the face-centered cubic (FCC) structure of silver, respectively (JCPDS file: 65-2871) (Devi et al. 2016b). It is noteworthy that found no other characteristic peaks in the obtained XRD spectra, which confirmed the high crystal phase purity of the formed Ag/AgCl NPs.

Fig. 2
figure 2

a XRD pattern, b TEM image, c FESEM image, d EDS pattern, and e FTIR spectrum for Ag/AgCl NPs synthesized by aqueous fruit extract of S. cumini

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TEM, FESEM, and EDS studies of S. cumini synthesized Ag/AgCl NPs

The TEM study was conducted to investigate the size, morphological features, and dispersion of the produced Ag/AgCl NPs, and the corresponding image is shown in Fig. 2b. TEM images revealed that the formed Ag/AgCl NPs were almost spherical with an average size of 16.9 nm. Also, the image displays that the synthesized Ag/AgCl NPs are well dispersed without significant agglomeration.

Further, FESEM images of the prepared nanoparticles confirmed the formation of spherical particles with a size between 20 and 24 nm (Fig. 2c), which matches the TEM images. The elemental analysis of the prepared Ag/AgCl NPs was done by using EDS measurement, as shown in Fig. 2d. The obtained results that show two peaks with high intensity were corresponding to Ag and Cl, which indicate the presence of Ag/AgCl. Thus, fruit extract acts as the chloride ion supplying agent as well as reducer and capping agents that are naturally available for the formation of Ag/AgCl NPs.

FTIR measurement

FTIR spectra were used to clarify the possible functional groups of phyto-compounds presented in the surface of the synthesized Ag/AgCl NPs that may be responsible for the reduction of Ag+ ions and stabilization of the formed Ag/AgCl NPs. The obtained FTIR spectra of S. cumini fruit extract and synthesized Ag/AgCl NPs are presented in Fig. 2e. The FTIR spectrum of S. cumini fruit extract indicates a broad and intense absorption peak at 3277 cm−1 related to stretching vibrations of the hydrogen-bonded O–H in alcohols and phenolic compounds such as flavonoids or amines (N–H group) (Devi et al. 2016b). The peaks at 2931 cm−1 and 2890 cm−1 were attributed to C–H stretching symmetric and antisymmetric modes of aliphatic and aromatic compounds, respectively (Gong et al. 2018). The presence of an intense absorption band in the range between 1750 and 1520 cm−1 could be attributed to C=C, carbonyl (C=O), or –N–H stretching vibrations of amide I and amide II, which is perhaps due to the presence of proteins/peptides that could act as stabilizing agents as well as reducing agents in the formation of Ag/AgCl NPs (Gong et al. 2018). The peaks at 1266 cm−1 and 1023 cm−1 can be indicators of the C–O and C=O of aromatic acids and esters, respectively (Durán et al. 2014).

The FTIR spectrum of the synthesized Ag/AgCl NPs demonstrates distinct peaks at 3202, 2918, 2878, 1621, 1407 and 1013 cm−1 (Fig. 2e). The comparison of the obtained spectra of the S. cumini fruit extract and the as-prepared Ag/AgCl NPs showed slight shifts in the peak positions as well as a decrease in the intensities. In particular, the shift of peak from 3277 to 3202 cm−1 may also indicate the involvement of the O–H functional group in the reduction of Ag+ ions and formation of Ag/AgCl NPs (Devi et al. 2016a). Also, the shift of peaks 2931 and 2890 cm−1 to 2918, 2878 cm−1, respectively, in the synthesized NPs indicates the involvement of bonds C–H of aliphatic and aromatic compounds in NPs synthesis (Patil et al. 2017). On the other hand, the appeared bands of fruit extract in the FTIR spectrum of the synthesized NPs revealed that phyto-constituents also protected the as-prepared Ag/AgCl NPs from aggregation (Devi et al. 2016a). From these results, it can be concluded that phyto-compounds in the S. cumini fruit extract drive the bio-reduction of Ag+ ions into silver nanoparticles (Ag0) and facilitate the formation and stabilization of Ag/AgCl NPs (Devi et al. 2016a; Gopinath et al. 2013; da Silva Ferreira et al. 2017). However, further studies are needed to confirm our results and to understand the exact role of S. cumini-derived bio-compounds in Ag/AgCl NPs production.

Sensing ability of the synthesized Ag/AgCl NPs toward clindamycin

The colorimetric sensing of an intended analyte is one of the most interesting methods because UV–Vis spectrophotometer is widely available and provides the naked-eye sensing of the analyte (Tavallali et al. 2013). By adding clindamycin to the diluted solution of the synthesized nanoparticles, the yellow color of the nanoparticles solution is changed to pink. From absorption studies, the SPR band of the synthesized Ag/AgCl NPs was shifted to red. This could be owing to the aggregation of nanoparticles in the presence of clindamycin and the decrease in the distance between nanoparticles. To achieve the maximum sensitivity of the proposed sensor to clindamycin, the pH of the solution, in the wide range of 3.0–11.0, and incubation time (in the presence of 14.0 × 10−5 M of clindamycin) were optimized and the obtained results are presented in Additional file 1: Fig. S2. The magnitude of the absorbance ratio of A500/A404 was used as a signal for clindamycin detection. According to Additional file 1: Fig. S2a, the pH of 3.0 showed better sensitivity and was selected in the following experiments. Moreover, Additional file 1: Fig. S2b shows the increase in the magnitude of A500/A404 from 1 to 3 min and after that became approximately constant. Therefore, the measurement was carried out 3 min after each injection.

Quantitative analysis was carried out by the injection of different concentrations of clindamycin into synthesized Ag/AgCl NPs solution, and the UV–Vis absorption spectra and color changes were recorded after 3 min of the addition of the drug. Figure 3 represents the SPR band of the synthesized Ag/AgCl NPs and its typical change during the addition of the different concentrations of clindamycin. The more increases in the concentration of the clindamycin resulted in more decrease in the absorbance at 404 nm accompanied by an increase in the absorbance at 500 nm. The linear range of the proposed method obtained from the relationship between A500/A404 (y = 0.0016x – 0.0019) and the concentration of clindamycin is shown in Additional file 1: Fig. S3. Good correlation coefficients (R2 = 0.99) and low detection limit (1.23 µM) with an acceptable linear dynamic range (over the range of 10.0–100.0 µM) are the main characteristics of the suggested method. These results represented the high efficiency of the synthesized Ag/AgCl NPs in the detection of clindamycin.

Fig. 3
figure 3

UV–Vis spectrum of diluted nanoparticles in the presence of different amounts of clindamycin and related color change as inset

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To evaluate the selectivity of the synthesized Ag/AgCl NPs toward clindamycin, uniform concentrations (35.0 µM) of several drugs (atenolol, antihistamine, cimetidine, glibenclamide, metformin, melatonin, gentamicin, ranitidine, erythromycin, clarithromycin, and rifampin) were added into the diluted solution of the synthesized nanoparticles. After a few minutes, the change in the color of the Ag/AgCl NPs solution was seen from yellow to pink only in the presence of clindamycin, and other drugs did not have any obvious color change. In Additional file 1: Fig. S4a and b, spectral changes and photograph images of the Ag/AgCl nanoparticles solution with different drugs were displayed, respectively. These results confirm that other tested drugs had little or no obvious effect on the color change and absorption spectra. Therefore, the proposed sensor has high selectivity toward clindamycin.

To survey the real sensing ability of the synthesized Ag/AgCl NPs in the detection of clindamycin, the sensing efficiency in pharmaceutical preparations (capsule and injection dosage) was done. The recovery of the proposed method was determined by spiking the clindamycin in two different real samples with known amounts of clindamycin, and the obtained results are presented in Table 1. The mean recovery values for clindamycin in the pharmaceutical samples were found to be 92.6–104.9% with relative standard deviation (RSD) values < 7%. Our study demonstrates that the proposed sensor could be acceptably applied in the quality control of clindamycin in the capsule and injection dosage forms.

Table 1 The amounts of spiked clindamycin into the capsule and vial of clindamycin samples
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In Table 2, the efficiency of our method is compared with several existing analytical methods for the determination of clindamycin in different matrices. Although the limit of detection (LOD) of most of the previous methods (Dedić et al. 2018; Habib et al. 2011; Kowalski et al. 2014; Liang et al. 2014; Liu et al. 2010; Pereira et al. 2018; Shao et al. 2006; Stanković et al. 2013; Szultka-Mlynska et al. 2018) is better than our method, most of these procedures have one or more disadvantages such as being laborious, time consuming, nonspecific, less safe, and too expensive. Our proposed method has the advantage of the high simplicity of optical sensors procedures without the need to perform any derivative reaction to convert the clindamycin into UV–Vis absorptive species. Furthermore, the synthesis of Ag/AgCl NPs is direct, one step, simple, economical, and eco-friendly without the addition of any hazardous reagents. Finally, with our proposed method, the determination of clindamycin in each sample is relatively fast.

Table 2 Dynamic range and LOD of the presented Ag/AgCl NPs sensor for determination of clindamycin and its comparison with other methods
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Sensing ability of the synthesized Ag/AgCl NPs toward Fe3+

The colorimetric sensing ability of the Ag/AgCl NPs towards various metal ions such as Li+, Na+, K+, Ba2+, Ca2+, Mg2+, Al3+, Bi3+, Cd2+, Co2+, Cu2+, Fe2+, Fe3+, Hg2+, Mn2+, Ni2+, Sn2+, and Zn2+, under the identical conditions, was explored in aqueous solution. In Additional file 1: Fig. S5a and b, the absorption band of the diluted Ag/AgCl NPs solution and its photographic images during adding of various metal ions are presented, respectively. Additional file 1: Fig. S5a displays that with the addition of diverse metal ions into the Ag/AgCl NPs solution, only Fe3+ and Hg2+ can result in discoloration. Owing to this considerable decrease in the SPR intensity, the facility of developing a sensitive procedure for the measurement of Fe3+ ions based on UV–Vis absorption spectroscopy has been investigated. The addition of other metal ions to the colloidal solution of Ag/AgCl did not depict a remarkable change in the color, demonstrating no significant interferences from other metal ions in the detection of Fe3+ ions. Although Hg2+ ions could also cause a significant decrease in the absorption intensity, according to previously reported studies (Basiri et al. 2018; Gao et al. 2015), this challenge could be circumvented by the addition of L-cysteine to the sensing solution as a masking agent of Hg2+. It is because of this fact that the Hg2+ has a major attitude to L-cysteine and could display a large sheltering effect to Fe3+ ions. Therefore, the selectivity of the proposed sensor toward Fe3+ probably could be improved.

In the following, the Ag/AgCl NPs assay for analytical detection of Fe3+ was optimized. The effect of the pH of the sensing solution and reaction time was studied. The pH condition in the detection of Fe3+ ions, using a colloidal solution of Ag/AgCl NPs in the presence of 4.0 × 10−6 M Fe3+ ions, was optimized in the wide range of 3.0–11.0, and the obtained results are displayed in Additional file 1: Fig. S6a. The change in the absorbance intensity, ΔA (ΔA = A0 − At, where A0 and At are the absorbance intensity of the Ag/AgCl NPs solution at 412 nm before and after the addition of Fe3+ ions, respectively), was applied as the analytical signal in the determination of Fe3+. Corresponding to Additional file 1: Fig. S6a, pH 9.0 was selected as an appropriate pH in the detection of Fe3+ ions. At pH of 9.0, more change in the color and spectral of Ag/AgCl NPs was noted. Also, Additional file 1: Fig. S6b represents the ΔA values from 1 to 5 min after the addition of Fe3+ ions. From Additional file 1: Fig. S6b, ΔA value became almost constant 1 min after the addition of Fe3+ ions, Therefore, the reaction was monitored 1 min after each addition of Fe3+ ions.

To calculate the linear dynamic range of concentration of Fe3+ ions in an aqueous solution, under the optimized conditions, the various concentrations of Fe3+ ions (10.0–930.0 µM) were added to Ag/AgCl NPs solution. From Fig. 4, as the concentration of Fe3+ ions increased, the SPR intensity gradually decreased, accompanied by a slight blueshift, and the sensing solution color changed from yellow to colorless (Fig. 4 inset). The blueshift suggests the reduction in the size of Ag/AgCl NPs in the presence of Fe3+ ions. In fact, with the addition of Fe3+ ions to the Ag/AgCl NPs solution, the Ag/AgCl NPs can be decomposed by Fe3+ ions, and its result is a gradual decrease in the SPR intensity dependent on Fe3+ concentration (Gao et al. 2015). The obtained linear range was 10.0–350.0 µM of Fe3+ (y = 0.002x – 0.0177, R2 = 0.9965) with LOD of 1.2 µM (S/N = 3, n = 3) (Additional file 1: Fig. S7). In Table 3, the comparison between different sensing systems for the determination of Fe3+ ions is summarized. The proposed sensing probe displayed an appropriate LOD and linear range as well as speciation toward Fe3+ ions without the interference effect of Fe2+ ions, compared with the other methods. On the other hand, the semiquantitative measurement of Fe3+ ions could be attained by naked eyes when the solution color changed.

Fig. 4
figure 4

UV–Vis absorption spectra of diluted nanoparticles solution in the presence of different amounts of Fe3+ ion and related color change as inset

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Table 3 Dynamic range and LOD of the presented Ag/AgCl NPs sensor for determination of Fe3+ and its comparison with other nanoparticles-based colorimetric methods
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To investigate the feasibility of the proposed sensing method, the standard addition procedure was applied to detect Fe3+ ions in water samples. First, the concentration of Fe3+ was determined in the investigated water samples (mineral and tap water) using atomic absorption with Flame atomization. Different concentrations of Fe3+ ions standard solution were added into mineral and tap water samples. After adding the desired real water to the designed sensing system, no change was observed in the yellow color of the mixture solution with no significant difference between the utilized blank solution and the added water samples in the UV–Vis absorbance. Hence, it was assumed that the Fe3+ ions concentration of water samples is zero or undetectable. The obtained results by the proposed method are shown in Table 4. Mean recovery values for Fe3+ ions in the spiked samples showed the accuracy of the proposed sample which was in the range of 90.5–109.5% and the RSD values lower than 6% confirmed its precision. According to the results, the suggested approach can be a low-cost way and portable platform for sensing Fe3+ ions in environmental water based on bioengineered Ag/AgCl NPs with great potential in selectivity and accuracy.

Table 4 Fe3+ ion recovery rates added in tap and mineral water samples
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Conclusion

Ag/AgCl NPs were synthesized via a one-step, simple, eco-friendly, and economical route by fruit extract of S. cumini without taking recourse to any surfactant and halide source. The effect of various parameters on the production of Ag/AgCl NPs was optimized. The optimal conditions were determined to be: pH = 7.0, the quantity of the leaf extract = 3.0 mL, 25 mL silver precursor with the concentration of 1.0 mM, temperature = 60 °C, and incubation time = 40 min. The synthesis of Ag/AgCl NPs was endorsed by monitoring the color change of the reaction media and showing the SPR at 412 nm. FTIR analysis revealed that the phyto-compounds of the fruit extract may be responsible for the production and stabilization of Ag/AgCl NPs. Also, the characterization of the synthesized Ag/AgCl NPs was performed by XRD, TEM, FESEM, and EDS analysis. Finally, the design of a new colorimetric sensor for the efficient distinction and identification of clindamycin and Fe3+ ions via phyto-synthesized Ag/AgCl NPs was done. The proposed method has eminent advantages including no pretreatment or derivatization, simplicity, high sensitivity, specificity, and analytical efficiency. This novel method can be used in the throughput analysis of clindamycin (in capsules and injection ampoules) and Fe3+ ions (in water samples).