Quantum dots (QDs) possess attractive optical properties, including broad excitation and narrow emission spectra,sizetunable emission profiles,high photoluminescence quantum yields,and excellent photochemical stability ^[1]. These unique properties of QDs show considerable advantages over traditional organic fluorophores ^[2] in the application of analytical chemistry, resulting in the increased use of QDs in various areas ^{[3, 4, 5, 6, 7, 8]}. Chen and Rosenzweig ^[5] reported for the first time that CdS QDs modified with different ligands could be used as luminescent probes for the determination of Zn²⁺ and Cu²⁺. Subsequently, dozens of QDs-based fluorescent methods were reported for the detection of various heavy metals ^{[6, 7, 8]}.

Most of the reported methods were based on the quenching or enhancement of fluorescence intensity of QDs upon the addition of heavy metal ions. They exhibited distinct advantages,including high sensitivity and low cost. However,these methods often suffered from interference caused by co-existing ions. In order to alleviate the interference,QDs functionalized with diethyldithiocarbamate, dithizone,and xylenol orange were synthesized and used recently for the determination of Cu²⁺ and Pb²⁺ in our laboratory ^{[9, 10, 11]}.

In this study,a novel strategy was proposed for the selective detection of Ag⁺,which was based on the red-shift in emission wavelength of QDs upon the addition of Ag⁺. Core-shell CdSe/CdS QDs functionalized by rhodanine (Rd) were synthesized and used as the fluorescence probe. Rd is often used as the silver reagent in molecular absorption spectrophotometry due to its specific and strong Ag⁺ binding capability. The objective of this study is to develop a sensitive and selective method for the detection of silver ions by exploring the unique properties of QDs and the specificity of Rd. 2. Experimenta

Fluorescence spectra were acquired on an F-2500 fluorescence spectrophotometer (Hitachi,Japan). AU-2810 spectrophotometer (Hitachi,Japan) was used to obtain ultraviolet-visible absorption spectra. FTIR spectroscopic measurements were carried out using Nicolet FTIR-6700 spectrophotometer (Nicolet,Japan). An Allegra 64R high-speed refrigerated centrifuge (Beckman,USA) was used to separate particles from solution. The pH values were measured by a Delta 320 pH-meter (Mettler Toledo,Switzerland). An AA240FS atomic absorption spectrometer (Varian,USA) was used in the verification test. Deionized water was produced by a Milli-Q system (Millipore,USA) and used for preparing solutions. All chemicals used were purchased from Sinopharm Chemical Reagent Co. and were of analytical-reagent grade,except for nitric acid which was of MOS grade.

Rd-functionalized QDs were synthesized in two steps. In the first step,the core-shell CdSe/CdS QDs capped by thioglycolic acid (TGA) were prepared according to a previously reported method ^[12]. In the second step,Rd-functionlized QDs were prepared by mixing 5 mL of 250μmol/L rhodanine ethanol solution and 5 mL of 1.2μmol/L TGA-QDs in a phosphate buffer solution (PBS,pH 8.0). The mixture was stirred for 4 h,allowing Rd to be attached to the QDs through the coordination between Rd and cadmium on QDs surface. The QDs were separated from the bulk solution for purification by precipitation of the particles with acetone and centrifugation. The separated QDs were redispersed in 10 mL of pH 8.0 PBS solution and stored in dark at 4℃ for further use.

For the detection of Ag⁺,100 μL of standard,or sample solution, and 200 μL of the synthesized QDs were added into 700 μL of 20μmol/L PBS (pH 8.0) and mixed for 20 min at 25℃. The fluorescence intensity and the emission wavelength were then measured at an excitation wavelength of 380 nm. 3. Results and discussion

Rhodanine on the QDs surface played an important role in the present study. To investigate whether rhodanine was bonded on the surface of the core-shell CdSe/CdS QDs capped by TGA,the FTIR study of Rd-functionalized QDs was conducted. As shown in Fig. 1(a),the IR absorption bands at 3130 cm^-1,1434 cm^-1 and 802 cm^-1 could be attributed to N-H group vibration of rhodanine.These bands were not observed in Fig. 1(b),indicating that the N-H group of rhodanine had been replaced by cadmium to form Rd-Cd complex. On the other hand,the IR characteristic peak of C=S of rhodanine was shifted from 1058 cm^-1 (Fig. 1(a)) to 1006 cm^-1(Fig. 1(b)),due to the formation of the Cd-S bond between sulphur of C=S of rhodanine and cadmium on the QDs surface. The strong vibration band of the O-H group at 3446 cm^-1 in Fig. 1(c) and 3458 cm^-1 in Fig. 1(b) clearly indicated that TGA had still remained on the QDs surface rendering the Rdfunctionalized QDs water-soluble. The absence of the N-H related peaks in Fig. 1(b) and the shift of the C=S peak provided the evidence of the formation of Rd-Cd complex and confirmed rhodanine had been bound to the surface of QDs.

Fig. 1

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Fig. 1.FT-IR spectra of free rhodanine (a),Rd-functionalized QDs (b) and TGA-QDs(c).

In theory,the more rhodanine molecules were bonded on the surface of QDs,the more complexation sites for Ag⁺ were introduced and thus,higher sensitivity could be achieved for Ag⁺ determination. However,increasing the amount of rhodanine could cause higher quenching of the fluorescence intensity and lower solubility of the Rd-funtionalized QDs. When the concentration of rhodanine reached 350μmol/L,precipitation was observed in the solution. Thus,a solution of 250μmol/L rhodanine in ethanol was used here for the preparation of Rd-functionalized QDs. The resulting Rd-functionalized QDs exhibited high optical stability without obvious decrease of fluorescence intensity or shift of emission wavelength over 3 months.

Fig. 2(a) shows the fluorescence spectra of the Rd-functionalized QDs in the presence of Ag⁺ with different concentrations. An obvious red-shift of the emission wavelength was observed upon the addition of Ag⁺. The red-shift of the emission wavelength can be attributed to the coordination between Ag⁺ and rhodanine on the QDs surface ^[13] and,subsequently,the increaseof theparticle size ^{[2, 14]}.Toinvestigate themechanism, the ultraviolet absorption spectra of the Rd-functionalized QDs were examined after adding various amounts of Ag⁺. As shownin Fig. 3,the spectra displayed a gradual red-shift with the increase of Ag⁺ concentration,indicating the formation of Rd-Ag⁺ at the Rd-functionalized QDs surface and the increase of the particle size ^{[15, 16]}.

Fig. 2

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Fig. 2.Fluorescence spectra of Rd-functionalized QDs after adding different concentrations of Ag+ (a) and the linear relationship between the red-shift of emission wavelength of the QDs and Ag+ concentration (b). The concentrations of Ag+ added for spectrum (1) to (11) were 0,0.0125,0.075,0.1,0.5,1,2.5,5,7.5,10,12.5 μmol/L,respectively.

Fig. 3

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Fig. 3.Absorption spectra of Rd-functionalized QDs in the presence of various concentrations of Ag+. The concentrations of Ag+ added for spectrum (1) to (5) were 0,1,2.5,7.5,10 mmol/L,respectively.

To further confirm the mechanism,Ag⁺ was added to the solution containing QDs without functionalization by rhodanine. Very little change of emission wavelength was observed when various amounts of Ag⁺ were added. Moreover,the possibility of agglutination of QDs,which might cause red-shift of emission wavelength,was also examined. A linear relationship was observed and that the linear relationship existed between the fluorescence intensity and the concentration of the QDs-Rd-Ag⁺ conjugates in the range of 20-150 nmol/L,indicating that the functionalized QDs were still mono-dispersed and uniform in aqueous solution ^[17]. The agglutination effect was thus ruled out. Therefore,it could be deduced that the coordination between Ag⁺ and rhodanine on the QDs surface induced the increase of particle size,which resulted in the red-shift of the emission wavelength.

The effect of pH on the red-shift of the emission wavelength was also investigated. As shown in Fig. 4,the red-shift of the emission wavelength was pH-dependent. The red-shift increased with the increase of pH value and reached a maximum at pH 8.0. Little change of the emission wavelength was observed at lower pHs, possibly owing to the competition of hydrogen ions with silver ions for reaction with rhodanine ^[18]. The decrease of the red-shift in emission wavelength at pHs greater than 8.0 was probably due to the precipitation of silver as silver hydroxide by reaction with hydroxyl ions ^[19]. The effect of pH was agreed well with the mechanism that the red-shift resulted from the coordination of Ag⁺ and rhodanine on the surface of the QDs. Therefore,pH 8.0 was chosen for subsequent studies.

Fig. 4

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Fig. 4.Determination of silver in environment samples.

The Rd-QDs concentration affects sensitivity and the linear range of the system. Table 1 shows the effect of concentration of Rd-QDs on the linear equation. As the concentration of the Rd-QDs increased,the linear range of the calibration plot increased,while the sensitivity decreased. Therefore,a concentration of 0.12μmol/ L of Rd-QDs is recommended as a compromise between sensitivity and the linear range.

Table 1

Table 1 Effect of concentration of Rd-QDs solution to linear equation.

Table 1 Effect of concentration of Rd-QDs solution to linear equation.

Table 2 shows the interfering effect of some common foreign ions,indicating the developed method is generally free from interferences. Transition metal ions,particularly Pb²⁺, Cu²⁺ and Hg²⁺,which were often reported to cause severe interference in QDs-based methods,exhibited negligible adverse effects over the tested concentrations. This was due to the specific and strong Ag⁺ binding capability of rhodanine introduced onto the QDs.

Table 2

Table 2 Interfering effect of other ions.a

Table 2 Interfering effect of other ions.a

As shown in Fig. 2(b),a linear relationship was observed between the red-shift of emission wavelength of the Rdfunctionlized QDs (y) and the concentration of silver ions (x) in the range of 0.0125-12.5μmol L^-1. The regression equation was as follows: y = 1.996x + 0.6067 (R² = 0.9988). The detection limit was evaluated using 3 σ/S and found to be 2 nmol/L,where σ was the standard deviation of the blank signal and S was the slope of the calibration plot. The US Environmental Protection Agency reported that the maximum contaminant level for total silver in drinking water was set to 0.9μmol/L. As soon as the content of silver exceeds the standard,it can be detected by our system. The relative standard deviation of 1.7% was obtained by 11 repeated determinations of 5μmol/L Ag⁺. It can also be seen in Table 3 that the present method had higher selectivity than the reported luminescent methods for detection of silver ions using QDs.

Table 3

Table 3 Comparison of the QDs-based fluorescence sensors for Ag+ determination.

Table 3 Comparison of the QDs-based fluorescence sensors for Ag+ determination.

The practical feasibility of the developed method was tested on several environmental samples with the results shown in Table 4. The recoveries of the spiked samples and the relative standard deviations (n= 3) were generally satisfactory. The results obtained by the developed system were in good agreement with those by the AAS method.

Table 4

Table 4 Determination of silver in environment samples.

Table 4 Determination of silver in environment samples.

A new method was developed and used successfully for the determination of silver in environmental samples based on the red-shift of the emission wavelength of Rd-functionalized QDs. The red-shift was attributed to the increase of particle size due to the coordination between Ag⁺ and rhodanine on the QDs surface. Compared with the reported methods for silver determination based on quenching,or the enhancing of fluorescence intensity of QDs,this method showed excellent selectivity based on the specific and strong affinity of Rd-Ag⁺ and unique properties of QDs. The principle of detection can be possibly extended to the analysis of other ions if appropriate chelating reagents are bonded to QDs. Acknowledgment

The National Natural Science Foundation of China is thanked for financial support (Nos. 20345006 and 20575043).