3.2 Fluorescence studies
In order to assess the ability of the synthesized probe to detect various cations, the fluorescence intensity of a 3 mL solution containing less than 10 ppm of probe was examined before and after adding 50 µL of different cations at an excitation wavelength of 215 nm. Figure 2 demonstrates that the presence of silver significantly increases the intensity of probe, while other cations have minimal effect on its intensity.
The measurement of silver ions was effectively conducted even in the presence of other cations within an aqueous medium, with the ratio of other cations to silver ions being 10 times higher. Figure 3 illustrates the disparity in peak intensities between the suspension solution and the presence of Ag+ alongside other ions.
To achieve the highest signal-to-noise ratio and sensitivity for silver (Ag+) determination, various variables such as probe concentration, contact time, and pH were investigated. Different amounts of NH2-HCP were added to 100 mL of deionized water and sonicated for 20 minutes. Subsequently, the fluorescence intensity of a 3 mL probe solution was measured in the presence of 50 µL of a silver solution (0.01 M). It was observed that even 0.01 g of the dispersed and sonicated probe resulted in a suspension that is inappropriate for fluorescence studies. So 0.001 g of the probe was dissolved and sonicated and this solution was selected for the studies.
The impact of contact time on fluorescence intensity was examined by adding 50 µL of a silver solution (0.01 M) to a 3 mL probe solution. Changes in fluorescence intensity were recorded at intervals of 30 seconds for a duration of 30 seconds to 3 minutes. The intensity remains constant immediately after mixing the reactants and forming the NH2-HCPAg complex. Generally, the fluorescence intensity does not exhibit significant variations over time once the reaction mixture is agitated, hence the probe possesses quick response time. Fluorescence intensity Vs. time chart is available on supplementary information.
The pH of the system was varied from 2 to 8, and the resulting impact on fluorescence intensity was monitored. For this purpose, 50 µL of the silver solution (0.01 M) was added to a 3 mL aqueous probe solution, and the fluorescence intensity was measured after 30 seconds. According to the results presented in Fig. 6, the optimum pH was found to be 3. Typically, pH strongly influences the fluorescence intensity of compounds containing nitrogen as nitrogen can exist in protonated or deprotonated forms. In alkaline solutions, there is competition between deprotonated form of the nitrogen ion and cations to occupy vacant probe sites. Additionally, metal hydroxide formation and silver hydroxide precipitation increase in alkaline solutions, so alkaline pH is not favorable for the study. In acidic solutions (close to neutral), as the group containing nitrogen protonates, the ability of silver to coordinate is reduced. Similarly, increasing the pH to neutral pH, protonates the nitrogen group of meta-phenylenediamine, weakening the interaction between silver and the probe [28]. So, the probe’s pH itself (pH = 3) was chosen for the rest of the studies. Figure 4 illustrates effect of pH on the response of the probe.
To determine the performance of the probe under optimized conditions, the following steps were taken: Initially, a concentrated silver solution with a concentration of 10− 4 M was added to the probe solution. A calibration curve (Fig. 5,6) was constructed by plotting the fluorescence intensity against the concentration of silver. The limit of detection (LOD) and linear range were determined by repeating the titration three times, resulting in values of 0.001 µM and 0.1-3 µM, respectively.
3.3 Mechanism of fluorescence enhancing
The increase in fluorescence observed in this study when Ag+ ions are introduced to the probe can be attributed to the metal-enhanced fluorescence mechanism. This means that the formation of Ag nanoclusters can amplify the local incident field surrounding the probe (referred to as the "incident field of the probe [29]). It is well known that metals like Ag and Au can enhance the fluorescence of certain fluorophores [30]. Therefore, the proposed mechanism is as follows: When Ag+ ions are added to the probe solution, an oxidation-reduction reaction takes place on the surface of the probe. This reaction leads to the reduction of Ag+ to Ag0 (metal Ag) and the oxidation of the probe. The interaction between the probe and the resulting small silver nanoclusters intensifies the emission of the probe [31]. It is unclear whether the fluorescence observed is solely due to the Ag nanoclusters, as they cannot be separated from the probe. However, the Ag nanoclusters in this case are not covered by organic ligands, indicating that their fluorescence characteristics differ from those of previously reported fluorescent monolayer-protected metal clusters [32–35]. The exact mechanism responsible for the enhancement of fluorescence requires further investigation. Nonetheless, previous studies have reported the formation of metal nanoparticles or nanoclusters in situ on carbon materials, including Ag nanoclusters on Carbon nanodots [36], Ag nanoparticles on cysteine [37], palladium nanoparticles on C-nanodots [38], and Au and platinum nanoparticles on carbon nanotubes [39], among others.