Introduction

Silver is widely used in the electronics, photosensing, and electroplating industries (Singha et al. 2015; Liu et al., 2008; Zhou et al. 2021). The overuse of silver has triggered a serious environmental crisis (Velmurugan et al. 2014). Ag+ is one of the most toxic metal ions for aquatic organisms because the accumulation of Ag+ can inactivate sulfhydryl enzymes and combine with imidazole carboxyl groups and amines in various metabolites. Thus, the detection and analysis of Ag+ is an important topic for health, epidemic prevention, and environmental monitoring. There are many ways to detect heavy metal ions, such as stripping voltammetry Xiao et al. 2010, atomic absorption spectrometry Luidmila et al. 2017, and inductively coupled plasma mass spectrometry Spokoyny et al. 2009. All of these methods are nonportable, expensive, and time-consuming (Lotfia et al. 2017). Fluorescence analysis, a highly sensitive and selective determination method, has developed rapidly in recent years (Wang et al. 2019). This method can not only be used to overcome the above shortcomings but also solve serious drawbacks, such as low sensitivity, hydrophobicity, low selectivity, and toxicity (Vaishnav and Mukherjee 2019; Jiang et al. 2018; Wang et al., 2019). Moreover, Ag+ is an effective fluorescence quenching agent for heavy metal ions. If appropriate substances are selected, fluorescence quenching for detecting trace Ag+ can be established.

As long as appropriate substances are selected, fluorescence quenching for detecting trace silver ions has been established in recent years (Djerahov et al. 2016). A few sensors, including sulphur atoms, have been reported for silver ion determination in recent years (Prasannan and Imae 2013; Park et al. 2015). However, many of these sensors have poor biocompatibility and poor water solubility, which hinders their application in water and biological environments. Therefore, the development of biocompatible and water-soluble fluorescent sensors for silver ions is one of the fields that scientists focus on. To date, a wide variety of fluorescent sensors have been developed, including metal nanoparticles, QDs and dye molecules, to solve the disadvantages of low sensitivity, low selectivity, insolubility toxicity and so on (Jiang et al. 2015). S QDs have attracted much attention because of their unique stability, low cytotoxicity and high biocompatibility. To date, S QDs as a new nanomaterial have played a vital role in many applications. It was found that there almost no fluorescent sensors are available to detect silver ions by S QDs (Lv et al. 2014). Therefore, it is necessary to develop high-sensitivity, green, portable, and low-cost S QD fluorescence sensors to detect silver ions.

Several coordination mechanism-based sensors for detecting ions have been exploited (Gao et al. 2015; Wang et al. 2007). Polymers have a tailorable and diversified structure and provide a new possibility for use as biosensors (Tabaraki and Nateghi 2016; Bian et al. 2016; Vaishnav and Mukherjee 2019; Jiang et al. 2018). However, most of these coordination polymers are macroscopic solid-state substances, which show very limited solution-based behaviour. These properties remarkably limit their application in biosensors.

We proposed a novel method to detect Ag+ with high sensitivity and excellent selectivity. Chitosan (CTS) and β-cyclodextrin (β-CD) was used to form a CTSCD polymer by formaldehyde, and the replicates of the experiment exceeded 85% (Binti et al. 2020). The cross-linked polymer solved the problems of poor biocompatibility and poor water solubility, and the reaction can be represented by a simple structure diagram as follows: S QDs combine with the amino group of CTSCD by an electrostatic effect to form S QDs/CTSCD nanocomposites. S QD/CTSCD nanocomposites rapidly aggregate in the presence of Ag+ because of their attraction to the surroundings of the S QD/CTSCD nanocomposites through electrostatic interactions. S–Ag+–S bonds are formed with S QD/CTSCD nanocomposites, leading to remarkable particle aggregation, and the fluorescence mechanism should involve aggregation-induced quenching (AIQ). A possible mechanism is shown in Fig. 1. To the best of our knowledge, this is the first study employing a facile self-assembly method to obtain S QD/CTSCD nanocomposites for Ag+ by an AIQ mechanism.

Fig. 1
figure 1

Possible mechanism for the S QD/CTSCD nanocomposite sensor operation

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Experimental

Reagents and instruments

All chemical reagents were purchased from Aldrich and used without further purification. Fluorescence was recorded using a Fluoromax-F7000 with a 10 nm slit, and UV–Vis spectra were obtained using Shimadzu UV-24500 equipment. X-ray photoelectron spectroscopy (XPS) was carried out by using an axisultra DLD electron spectrometer from Shimadzu. Fourier-transform infrared (FTIR) spectra were obtained by using a Nicolet Magna 550 spectrometer. The morphology of the S QDs was observed by a JEOL JEM 2100 transmission electron microscope.

Preparation of CTSCD composites

Exactly 3.000 g CTS was completely dissolved in 300.0 mL (0.1000 mol L−1) hydrochloric acid, and 15.00 g β-CD was dissolved in 600.0 mL distilled water. The above solutions were mixed in a 1000.0 mL beaker, heated to 333.0 K, and then slowly added to 14.40 mL (25.00%) formaldehyde. The temperature of the solution was increased to 363.0 K, and the reaction was carried out under stirring for 80.00 min. After the reaction was completed, NaOH was added dropwise into the solution until pH = 11.00, and a yellow precipitate was produced. The same temperature was maintained, and the reaction was continued for 45.00 min, filtered, and then washed with acetone and ethanol to neutral pH. The product was dried at 318.0 K until a constant weight was achieved (Binti et al. 2020). CTSCD composites (the scheme is shown in Fig. 2) showed a red–brown colour (the replicate rate of the experiment exceeded 85%).

Fig. 2
figure 2

The reaction scheme used to prepare CTSCD composites

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S QD-functionalized CTSCD composites

Exactly 0.04 g CTSCD was dissolved in 6.00 mL anhydrous ethanol. S QDs (1.4852 mL (3.00 mg/mL)) were added into the solution. The mixture was reacted for approximately 30 min by ultrasound, and S QD/CTSCD nanocomposites were obtained.

Fluorescence properties of S QD/CTSCD nanocomposites

The fluorescence sensing properties of S QD/CTSCD nanocomposites were determined by adding a fixed amount of S QD/CTSCD nanocomposites (500 μL) to samples containing varying Ag+ concentrations, and the corresponding fluorescence spectra were recorded. The Ag+ content in the sample was determined as follows: Different concentrations of Ag+ were mixed with 500 μL S QD/CTSCD nanocomposites and 2.5 mL ultrapure water. After 5 min, the solution was subjected to luminous measurement at λ ex = 295 nm. The λ ex for the S QD/CTSCD nanocomposites was 295 nm in all tests, and the emission was monitored from 320 to 580 nm. The width of the excitation and emission slits was 5 nm.

The selectivity for Ag+ detection was investigated by preparing the same samples following the above method. Fe3+, Ca2+, Pb2+, Na+, Cr3+, Bi3+, Zn2+, Mg2+, Ba2+, Cd2+, Ag+, Sr2+, Hg2+, Ni2+, Ag+, and other solutions were added to each sample until the same concentration (10 μM) was reached. All mixtures were incubated at room temperature for 20 min.

Application to actual water samples

The practicability of the sensors was evaluated by detecting samples obtained from Xianyang Lake, Gudu River, and tap water at different places in Xianyang. Impurities were removed by filtration and centrifugation, and the Ag+ ions in the samples were detected by using the standard method. The detection procedure was the same as the above procedure.

Results and discussion

FTIR spectroscopy and DSC for S QD/CTSCD nanocomposites

Figure 3a shows the infrared spectra for the S QD, CTS/β-CD, and S QDs/CTSCD nanocomposites. The peak at 1640 cm−1 is a typical characteristic of S QDs (Wang et al. 2019), and the peak at 896 cm−1 is attributed to the β-(1, 4)-glycosidic bonds of CTS. The peak at 1042 cm−1 indicates the presence of the α-(1, 4)-glycosidic bond of β-CD. The peak at 1640 cm−1 corresponds to S QDs, indicating that S is successfully introduced into the CTSCD composites. The peaks at 1560–1640 and 2100 cm−1 are mostly attributed to amino (–NH2), alicyclic amine I (C=O), and cyclic amine II (N–H) groups. The absorption peak for the S QDs/CTSCD nanocomposite was reduced due to the reaction with the amino group. The absorption peak for the composite at 3440 cm−1 was enhanced because of the tensile vibration peak for –OH.

Fig. 3
figure 3

FTIR (a) and DSC (b) spectra for the S QD/CTSCD nanocomposites

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The DSC results for the CTS/β-CD and S QD/CTSCD nanocomposites are shown in Figs. 3b. The curve shows an evident endothermic peak at 97.25 °C, which is attributed to the crystalline water that evaporates from CTSCD. The melting peak for CTS occurs at approximately 225 °C due to the internal H bond, whereas the melting peak for CTSCD occurs at approximately 228 °C, indicating a greater stability compared to the CTS monomer (Djerahov et al. 2016; Teng et al. 2020a, b; Rao et al. 2020; Li et al. 2020; Li et al. 2020). From the curve for the S QDs/CTSCD nanocomposites shown in Fig. 3b, an evident endothermic peak is found at 100.30 °C. The results show that the thermal stability of CTSCD is improved because S QDs are successfully introduced into the CTSCD composites. The exothermic peak at 259.48 °C indicates the degradation peak for the S QD/CTSCD nanocomposite chain, which is due to the weakening of hydrogen bond interactions and the destruction of structure regulation by S QDs. The two lines in Fig. 3b show that S QDs improve the thermal stability of S QD/CTSCD nanocomposites by increasing the degradation temperature of the chain skeleton.

The optical properties and TEM for S QD/CTSCD nanocomposites

The optical properties of the S QD/CTSCD were studied by UV absorption and fluorescence spectra at room temperature. Figure 4a shows a 298 nm peak in the UV absorption spectrum. The ultraviolet absorption for S QDs, which is modified by the CTSCD polymer, is stronger than that of S QDs. Therefore, as shown in Fig. 4a, the S QD/CTSCD nanocomposite solution is light yellow (a) in sunlight but bright blue when irradiated with 365 nm UV light (b), indicating the blue fluorescence properties of the S QD/CTSCD nanocomposites. When excited at 295 nm, the fluorescence spectrum shows an excellent emission peak at approximately 411 nm, which is shown in Fig. 4b.

Fig. 4
figure 4

UV/Vis spectra (a) and fluorescence emission (b) for the S QD/CTSCD nanocomposites

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TEM micrographs (Fig. 5a) show a good dispersion of S QDs in water. Particles mostly show a regular spherical shape with a size of approximately 2–3 nm. A typical amorphous structure is observed with no visible lattice. The results show that S QDs have excellent nanoparticle properties. The atomic force microscopy (AFM) image shows the shape and height of the S QDs. The average height is 0.8 nm (Fig. 5b). The TEM image shows the good dispersion of S QD/CTSCD (Fig. 5b, c) with a relatively uniform size distribution.

Fig. 5
figure 5

a TEM image of S QDs with diameters of approximately 2–3 nm; b AFM representation of S QDs; and c TEM for the S QD/CTSCD nanocomposites

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XPS for S QD/CTSCD nanocomposites

The composition, surface group, and structure of the S QD/CTSCD were studied by XPS. The nanocomposites show four peaks at 165.9 eV, 284.30 eV, 398.8 eV, and 532.04 eV (Fig. 6a) representing S2p, C 1 s, N 1 s, and O 1 s in the XPS spectra, respectively. The XPS results show that the nanocomposites are primarily comprised of S, C, O, and N. The C 1 s spectrum (Fig. 6b) shows two peaks at 282.73 eV and 285.9 eV, which should be attributed to C–C and C–OH, respectively. Three peaks are found at 165.75 eV, 166.25 eV, and 165.5 eV in the S2p spectra (Fig. 6c), which are attributed to the SO2− (2p1/2), SO3− (2p2/3), and SO3− (2p1/2) bands, respectively. The two peaks at 395.9 eV and 397.1 eV in the N 1 s spectrum (Fig. 6d) are attributed to the C–N–C and C–N groups, respectively. The O 1 s spectrum (Fig. 6e) shows two peaks at 532.7 eV and 531.9 eV, which are due to the C–OH/C–O–C and C = O bands, respectively (Park et al. 2015; Teng et al. 2018; Wu et al. 2020). The results indicate that S QDs are already successfully introduced into CTSCD composites.

Fig. 6
figure 6

XPS for the S QD/CTSCD nanocomposites

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The effect of pH on the fluorescence properties

Figure 7 shows the effect of pH on the fluorescence properties of the S QD/CTSCD solution. In the experiments, 0.1 M HCl and NaOH were added to the solution to obtain the desired pH (4–10). In a strong acid environment (pH ≤ 4), the amino groups on chitosan protonate (Gao et al. 2014; Yang et al. 2020) and destroy the weak self-assembly between the CTSCD polymer and S QDs. S QDs can detach from the polymer chain, and the fluorescence intensity can increase significantly (pH = 4). With increased pH, the fluorescence intensity decreases by approximately 7%. This negligible change might be the result of a small change in the quantum confinement due to functionalization of the S QD/CTSCD. S QD/CTSCD samples did not show a change in their maximum fluorescence emission value (λ em) or shape. Under the experimental conditions, the S QD/CTSCD samples were monodispersed, and the fluorescence emission was stable (Dager et al. 2019).

Fig. 7
figure 7

The effect of pH on the fluorescence properties of S QD/CTSCD nanocomposites

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The selectivity of the sensor

Selectivity is an important parameter for assessing fluorescence properties. The effects of metal ions, such as Fe3+, Ca2+, Pb2+, Na+, Cr3+, Bi3+, Zn2+, Mg2+, Ba2+, Cd2+, Ag+, Sr2+, Hg2+, and Ni2+, at a fluorescence concentration of 1.0 × 10–5 mol/L were examined. As shown in Fig. 8a, the fluorescence intensity from S QD/CTSCD nanocomposites in the presence of Ag+ is quenched to a remarkable extent, whereas that in the presence of other metal ions (e.g., alkali, alkali soil, and transition metal ions) at 1.0 × 10−5 mol/L shows negligible changes or remains unchanged. Thus, a sensor based on S QD/CTSCD shows good selectivity towards Ag+.

Fig. 8
figure 8

The sensing properties of S QD/CTSCD nanocomposites (A, Selectivity; B, Sensing properties (silver ion concentration from 1×10−5 mol/L to 5.5×10−5 mol/L);C, Linear properties; D, S-V equation;C Ag+: 1,1×10−5 mol/L; 2,1.5×10−5 mol/L; 3, 2×10−5 mol/L; 4, 2.5×10−5 mol/L; 5, 3×10−5 mol/L; 6, 3.5×10−5 mol/L; 7, 4×10−5 mol/L; 8, 4.5×10−5 mol/L; 9, 5×10−5 mol/L; 10, 5.5×10−5 mol/L)

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The sensing properties of the S QD/CTSCD nanocomposite for silver ions

Under optimum conditions, the sensitivity of the S QD/CTSCD nanocomposite towards Ag+ was investigated. In a typical operation, the S QD/CTSCD solution (0.01 mg mL−1) was dispersed in water and mixed with varying amounts of Ag+. After the samples were incubated at room temperature for 10 min, the fluorescence emission spectra were recorded and analysed in triplicate. The fluorescence intensity was calibrated by changing the Ag+ concentration.

Figure 8b shows that the fluorescence peak for the S QD/CTSCD nanocomposites gradually decreases with increasing Ag + concentration because of the S–Ag+–S bonds formed between Ag+ and the S QD/CTSCD nanocomposites. Based on this phenomena, a favourable linear correlation exists (R2 = 0.9992) between I and Ag+ at concentrations ranging from 1 × 10−5 to 5.5 × 10−5 mol/L (Fig. 8c), and the detection limit for Ag+ based on the 3δ/slope was determined to be approximately 85 nM. The effectiveness of Ag+ fluorescence detection with S QDs/CTSCD as a fluorescence probe was verified, which provides a platform for Ag+ detection. The selectivity of S QD/CTSCD nanocomposites for Ag+ might be due to the synergistic effect of S and O functional groups on S QD/CTSCD nanocomposites (Nam et al. 2017). F0 is [C] = 0, where F is the fluorescence intensity before and after adding Ag+, KSV is the quenching effect coefficient of the sensing material, and [C] is the molar concentration of the analyte.

When Sb is a blank signal standard deviation, the Ag+ (C) concentration is linear with a decreased Ag+ (C) concentration in the range of 1 × 10−5 to 5.5 × 10−5 mol/L (I0/I = 1 + 0.0029C [R2 = 0.9969]). The concentration of silver ions ranges from 1.0 × 10−5–5.5 × 10−5 mol/L (Fig. 8d). The results show that the S QD/CTSCD nanocomposite is a more sensitive fluorescence sensor of silver ions compared to other sensors (Lu et al. 2014; Jiang et al. 2015). The sensitivity of the Ag+ sensor is higher than that of other methods reported in the literature (Lv et al. 2014; Gao et al. 2015; Wang et al. 2007). As shown in Table 1, the detection limit and the analytical concentration range for different technologies (e.g., electrochemical, high-performance liquid chromatography, and other methods) are much lower than that for the proposed sensor. Solid-phase microextraction, which is used as a detector in liquid chromatography-tandem mass spectrometry, is comparable with the proposed sensor. However, as reported in the literature, these processes are expensive, time-consuming, and inconvenient. Our sensor shows satisfactory properties in terms of analysis time, sensitivity, cost, and operation (on-site analysis) and can be used to detect Ag+ directly in actual samples. The quenching effect can be rationalized by the Stern–Volmer equation: I0 /I = 1 + 0.0029C (R2 = 0.9966).

Table 1 Comparisonof various fluorescence sensors used to detect silver ions
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3.7 Sensor mechanism

A simple method for synthesizing fluorescent S QD/CTSCD nanocomposites that possess fluorescence detection was presented. The functionalization of S QDs on the CTSCD structure is determined by the ability of the functional group. Ag+ serves as a coordinating centre that directly reacts with S QDs on the CTSCD chain. The hydration radius of Ag+ can exactly match the distance on the S QD-functionalized CTSCD structure. S QD-functionalized CTSCD supramolecular structures are promising because their structure contains potential coordination centres comprising metal cations. S QDs in the polymer structure form S–Ag+–S bonds with Ag+ ions, which combine with colloidal particles with noncovalent bonds and form a dendritic fractal structure of aggregates based on Ag+. The fluorescence mechanism should be aggregation-induced quenching (AIQ).

The effect of the metal ions

The recognition property of the sensor against various metal ions was explored by using the fluorescence response to Ag+. The effect of some coexisting cations on the detection of Ag+ is shown in Fig. 9. Most coexisting metal ions do not interfere with the binding of Ag+ to the S QD/CTSCD nanocomposites except for Ni2+ ions, which have a weak effect on detection. Therefore, the selective binding of Ag+ can be carried out in the presence of the most competitive and coexisting metal ions.

Fig. 9
figure 9

Metal ion interference during Ag + sensing by S QD/CTSCD nanocomposites (a: Fluorescent spectrogram; b: Histogram)

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Detecting silver ions in environmental samples

The sensing properties of S QD/CTSCD nanocomposites were analysed in river water (Xianyang Lake, Gudu River, and Nanhu Lake in Xianyang, China). Similar to the results obtained for distilled water, the fluorescence intensity of the S QD/CTSCD nanocomposites in river water drops after adding Ag+. Although there various substances are present in river water that can interfere with the detection of Ag+, the results show that the recovery of Ag+ in the sample is 98.44–110.76% (Table 2). The measured values show that the designed sensor is reliable and practical in different environmental water samples. Therefore, the S QD/CTSCD nanocomposites exhibit fluorescence properties that are sensitive and selective to Ag+.

Table 2 A comparison of the different samples used to analyse the detection of silver ions
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Conclusion

We demonstrated a facile strategy to synthesize S QD/CTSCD nanocomposites via self-assembly. The fluorescence from S QD/CTSCD nanocomposites can be sensitively and selectively quenched by Ag+ based on an aggregation-induced quenching mechanism. The advantages of these nanocomposites are summarized in the following aspects: (1) S-QDs are successfully linked to the CTSCD polymer chain through a simple and convenient ultrasonic reaction. S QD/CTSCD exhibit excellent fluorescence behaviour; (2) S QD/CTSCD show a good selective and sensitive response to Ag+; (3) A simple and direct method to detect Ag+ was realized with a fast response, wide linear range, and low detection limit; (4) Actual samples from the environment were analysed, and the results show that the error rate detection for Ag+ is less than 11%. The novel sensor is easily synthesized by a simple process and has broad application prospects in the field of sensing.