Introduction

In recent years, the development of fluorescent chemosensors for detecting biologically and environmentally important metal ions was a hot research subject in the communities of chemistry, biochemistry, biology, material science and others due to its single molecular detection sensitivity, the nondestructive, and real time monitoring capability [1, 2]. Among all these fluorescent sensors including fluorescence turn-off and turn-on sensors upon metal coordination, the fluorescence turn-on sensor is highly desirable to enhance the sensitivity [2–4]. In terms of sensitivity concerns, fluorescent sensors that exhibit a “turn-on” emission increase or cause a shift in excitation/emission profiles in response to cations are preferred over a turn-off emission-quenching response due to their better spatial resolution under a light microscope [5]. In contrast, fluorescence “turn-off” sensors may report false positive results caused by other quenchers in practical samples and are undesirable for practical analytical applications. Moreover, most of “turn-off” fluorescent chemosensors are driven by UV light excitation, which might suffer from the influence of background fluorescence [6, 7].

Although a number of polymer fluorescent sensors for transition metals including zinc [8], nickel [9], iron [10–13], chromium [10] and mercury [10, 14–16] have been developed in recent years, relatively few copper-selective sensors have been reported [17–20]. It is well known that, Cu2+ is a significant environmental pollutant and an essential trace element in biological systems, it is eager to design and synthesize novel sensors for the measurement and detection of copper ion [21–23]. However, due to the paramagnetic nature of the copper ion, some sensors often undergo fluorescence quenching upon the Cu2+ binding, subsequently compromising the sensitivity of the sensors [6]. Most of small molecule sensors are difficult to dissolve in water [18–20] or take on a turn-off emission [18, 20]. Thus, the development of turn-on fluorescent sensors with high sensitivity and selectivity for monitoring Cu2+ in aqueous medium remains a significant challenge.

Mercury is a prevalent toxic metal in the environment that poses a significant public health hazard. Both elemental and ionic mercury can be converted to methyl mercury by bacteria in the environment, and subsequently bioaccumulate through the food chain. Once absorbed in the human body, mercury damages the central nervous and endocrine systems. Because of the health concerns associated with mercury exposure, it is important to develop effective methods for detecting Hg2+ [24–27]. Hence, considerable attention has been devoted to the development of new fluorescent chemosensors for the detection of mercury and mercuric salts with sufficient selectivity [28].

Among the available fluorescent substances, rhodamine and its derivatives possess excellent photophysical properties due to its particular structural property [2]. As is well-known, the rhodamine moiety in a free molecule takes a spirolactam ring-closed form, which usually shows little absorption and nonfluorescence. However, it transforms to ring-opened amide form upon binding with a specific species, the chelating or reaction of metal ions, which usually becomes highly absorbent and fluorescent. Based on such structural character, many turn–on-type chromogenic or fluorogenic rhodamine-based chemosensors and reagents for metal ions were fabricated in the past few years [2, 29, 30]. The rhodamine derivatives have long absorption and emission wavelengths that extend to the visible region, high fluorescence quantum yields, large absorption coefficients, and the potential for turn-on fluorescence detection. All these attributes make rhodamine derivatives are good candidates for sensing Cu2+ or Hg2+ [24, 31]. To date, several rhodamine- modified chemosensors and probes based on the spirocyclic ring-opening mechanism have been developed [7, 23, 32–35]. Nevertheless, these rhodamine-based small molecule chemosensors typically exhibit poor water solubility and usually only function in a medium of pure organic solvent or an aqueous solution containing at least 50 % organic cosolvent. The lack of water solubility greatly limits the potential applications of rhodamine-based small molecules in biological systems and in environmental analyses [9, 36]. This inconvenience can be overcome by using hydrophilic copolymers that also contain small amounts of the lipophilic organic receptors. When designed correctly, the inclusion of the receptor moieties does not alter the hydrophilicity of the overall materials, thus permitting the preparation of water-soluble sensory copolymers [10]. In contrast with small molecular sensors, polymer sensors exhibit prominent advantages, such as the ease of fabrication of devices, a wide choice of incorporating specifc units into functional polymers, low cost, and so on [11, 37]. The copolymers of traditional monomers with some polymerizable fluorescent units display intensive fluorescence. Thus, using appropriate fluorophores, the different fluorescent polymeric sensors can be obtained [11]. On the other hand, photostability of polymers is of great importance for their use. The covalent bonding of the fluorophores to the polymer chain provide a good stability to solvents and migration, improving their environmental behaviors [17, 38, 39].

In our previous works, we have successfully designed fuorescence chemosensors for Cu2+or Hg2+ receptors utilizing rhodamine as the fluorophore [21, 27, 40–48]. As part of our continuous works on copper and mercury ions chemosensors based on the rhodamine derivatives, we report the synthesis and properties of a water-soluble polyacrylamide containing rhodamine 6G (poly(AM-AR6GHS), see Scheme 1) and investigate the selective sensing ability.

Scheme 1
scheme 1

Synthetic route of poly(AM-AR6GHS)

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Experimental

Reagents and instruments

Rhodamine 6G (99 %), 2,2′-azobis(2-methylpropionitrile) (AIBN, 98 %) were purchased from Acros organics; Hydrazine hydrate (85 %, AR) was purchased from Sinopharm Chemical Reagent Co. Ltd; Acryloyl chloride, acrylamide (AM), salicylaldehyde, triethylamine and acetic acid were supplied by Aladdin Chemistry Co. Ltd. All above were used without further purification. Anhydrous methanol, ethanol, trichloromethane, dichloromethane, and tetrahydrofuran were obtained from commercial suppliers and were purified by distillation before use. All aqueous solutions were prepared using double distilled water from a quartz automatic double distilled water distillatory 1810-B system (Baita quartz glass instrument factory, Jintan, Jiangsu). Rhodamine 6G hydrazide (R6GH) and R6GHS were synthesized according to literatures [48–54].

1H NMR experiments were performed on a 400 MHz (AVANCE AV 400, Bruker corporation, Switzerland) with CDCl3 and D2O as solvents. FTIR spectra were recorded using KBr pellets on a AVATAR360 FTIR spectrophotometer (Nicolet corporation, America) in the 4000–400 cm−1 regions. Elemental analyses (C, H and N) were carried out on a VarioELIII analyzer (Elementar corporation, Germany) for the monomer AR6GHS. All pH measurements were made with a Model pHS-3C meter (Shanghai, China). Fluorescence emission spectra were recorded on a RF5301PC fluorescence spectrophotometer (Shimadzu Corporation, Japan). The viscosity average molecular weight (Mη) was estimated for PAM and poly(AM-AR6GHS) from intrinsic viscosity of the polymers in aqueous solution at a constant temperature of 30 °C with Ubbelohde viscometer (Φ = 0.51 mm).

Measurement procedures

The poly(AM-AR6GHS) stock solution (1.00 mg/mL) was prepared in an aqueous solution. The pH was adjusted by the addition of H2SO4 or NaOH to the copolymer aqueous solution. The pH of the solution was adjusted from 2 to 12 by a digital pH controller. The effect of the metal cations on the fluorescence intensity was examined by adding a few microlitre of the metal cations solution (the concentration of metal cations were 0.10 mol/L) to a known volume of the copolymer solution (2.00 mL). The total volume of the addition metal cations solution was limited to 0.10 mL, so that the dilution of the copolymer solution remained insignificant. The metal ions are nitrate salts of Ag+, Ba2+, Cd2+, Co2+, Cu2+, Fe2+, Hg2+, K+, Mg2+, Ni2+, Pb2+, Zn2+, Cr3+ and Fe3+, whose solution were prepared in double distilled water. For all fluorescent measurements, excitation wavelength was 500 nm with the emission recorded over the wavelength range of 520–650 nm [16]. The detection limit was calculated with the equation: detection limit = 3S/ρ, where S is the standard deviation of blank measurements and ρ is the slope between intensity versus sample concentration.

The viscosity average molecular weight (Mη) of PAM and poly(AM-AR6GHS) was estimated from intrinsic viscosity of the polymers in distilled water at a constant temperature of 30 °C, using the Mark–Houwink–Sakurada (MHS) equation [η] = kMη α (K = 9.33 × 10−3, α = 0.75 for the polymers) [55, 56].

Synthesis of AR6GHS

Methylene chloride (20 mL), 0.5326 g (1.0 mmol) of R6GHS, and triethylamine (0.54 mL) were added into 100 mL three-neck round-bottomed flask. The mixture was placed in an ice bath and agitated vigorously using an overhead stirrer under a nitrogen atmosphere. When the temperature dropped below 10 °C, acryloyl chloride (0.1104 g, 1.22 mmol) in methylene chloride (10 mL) was added slowly from an addition funnel such that the temperature was maintained below 10 °C. The mixture was stirred for an additional 24 h at 25 °C after the addition of acryloyl chloride. The methylene chloride was evaporated. The yellow viscous solid was washed with NaOH (0.10 mol/L, 25 mL × 2), water (10 mL × 2) and anhydrous ethanol (10 mL × 2), respectively. The ashen powder was collected throuth suction filtrated, and dried under vacuum at 50 °C for 24 h (yield: 65.09 %) [11, 53]. FTIR(KBr) of AR6GHS, ν/cm−1: 3425.20 (-N-H); 3061.07, 3019.69, 854.42 (C=CH2); 2978.31, 2936.93, 2868.46 (-CH3, -CH2-); 1713.61(C=O ester group), 1686.52 (C=O, amide group); 1631.60, 1514.23, 1425.45, 820.56,723.03 (aromatic C=C); 1274.23, 1219.31(C-O). 1H NMR of AR6GHS (400 MHz, CDCl3), δ: 1.114–1.185(s,–CH2-CH 3); 1.915 (s, -Φ-CH 3); 3.133–3.185 (m, -Φ–CH 2 -CH3); 3.630 (m, -Φ–NH-CH2); 5.749–5.773 (d, CH=CH 2); 6.228–6.251 (m,–Φ-H); 6.490–6.513(m, CH2=CH-); 6.702-6.720 (m, −Φ-H); 6.734–6.773 (m, –Φ-H); 7.923-7.923 (d, −Φ-H); 7.945−7.933(m, −Φ-H); 9.087(s, Φ-CH=N-). Anal. calc. for C36H34N4O4 (%): C, 73.71; H, 5.84; N, 9.55. Found (%): C, 73.86; H, 5.95; N, 9.32.

Preparation of PAM and poly(AM-AR6GHS)

A solution of acrylic amide (0.7108 g, 10.0 mmol) and AIBN (8.2 mg, 0.05 mmol) in 15 mL of dry THF was introduced into a dry three-neck round-bottomed flask. The solution was deoxygenated by purging with purified N2. The three-neck round-bottomed flask was sealed and placed in a regulated thermostat bath at 65 °C for 12 h. The solution was cooled to room temperature and then allowed to precipitate in excessive methanol. The white precipitate was collected by filtration and dried under vacuum to a constant weigh (yield: 95.29 %) [11].

Poly(AM-AR6GHS) was prepared by the copolymerization of AR6GHS and AM with AIBN as initiator by similar methods as the polymerization of AM. Under nitrogen condition, AM (0.3518 g, 4.95 mmol), AR6GHS (0.0293 g, 0.05 mmol) and AIBN (4.1 mg, 0.025 mmol) were dissolved in 7.5 mL of THF. The above solution was heated to 65 °C for 12 h. After cooling to room temperature, the solution added dropwise into 50 mL of methanol to obtain poly(AM-AR6GHS), which was purified by precipitating three times in methanol. Pale pink powder was obtained after filtration and then dried at 50 °C under vacuum overnight (yield: 87.20 %) [11]. FTIR(KBr) of poly(AM-AR6GHS), ν/cm−1: 3404.89, 3205.52 (-CO-NH2); 2923.39, 1452.54 (-CH2-,); 2854.92 (-CH-); 1672.98 (C=O); 1617.30, 1452.54, 1425.45, 1198.24 (aromatic C=C); 1123.01(C-O). 1H NMR of poly(AM-AR6GHS) (400 MHz, D2O), δ: 1.137–1.169 (m, -CH3); 1.598 (d, -CH2-); 2.159–2.279 (m, -CH-); 3.672–3.690 (s, -CH2-); 3.705–3.798 (m, -Φ-CH-CH 3); 2.661 (m, -Φ-CH3); 4.559−5.057 (m, -CO-NH2); 6.139–6.270 (s, -Φ–H); 6.913−6.270 (m, Φ-H) 7.710(s, -Φ–H). The Mη for PAM and poly(AM-AR6GHS) were 6.09 × 104 and 5.72 × 104 g/mol, respectively.

Results and discussion

Design and synthesis

The synthetic strategy of the monomer and copolymer was shown in Scheme 1. As shown in Scheme 1, the polymerizable monomer (AR6GHS) was synthesized using R6G as the starting material. The rhodamine 6G hydrazide (R6GH) was prepared in a reaction between R6G and hydrazide. The intermediate species R6GHS was produced by a reaction of R6GH with salicylaldehyde. The AR6GHS was then synthesized by a reaction of R6GHS with acryloyl chloride, whose structure was confirmed by FTIR, 1H NMR and elemental analyses. The monomer AR6GHS which is a Shiff base containing amide carbonyl oxygen was designed to chelate with metal ions [2, 6, 7, 17, 19, 21–23, 30, 35, 48, 53]. The AR6GHS could chelate with metal ions via its N of imino and O atoms of carbonyl or phenol. Due to the good stability of the complexes formed by the Schiff base ligands with Cu2+ or Hg2+, the fluorescent sensors based rhodamine Schiff base ligands have attracted a great deal of attention in recent years [23, 33, 34].

The polymerization of AR6GHS and AM was carried out in THF with AIBN as initiator at 65 °C under N2 atmosphere [57]. It is well known that small molecular derivatives of R6G has a poor water solubility, which limited its applications. In order to avoid these disadvantages, we used the water soluble AM to copolymerize with AR6GHS. As expected, the resulting copolymer was water-soluble [11]. The spirolactam moiety in the rhodamine group could acted as a signal switcher, which was envisioned to turn on when the cation was bound [49]. Upon the addition of Cu2+or Hg2+ ions, the spirolactam moiety of the rhodamine group opened (the formation of an open-ring structure), which resulted in the appearance of fluorescence (Scheme 2) [9, 58].

Scheme 2
scheme 2

The diagrammatic sketch of complex of poly(AM-AR6GHS) with Cu2+ or Hg2+ cations

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Structure of poly(AM-AR6GHS)

FTIR and 1H NMR spectroscopies were used to confirm the linkage between the R6G moiety and the polymer. The peaks at 3061 and 3020 cm−1 which was corresponding to the double bond of the monomer AR6GH were disappeared after polymerization as presented in the FTIR curve of poly(AM-AR6GHS). In contrast, the new characteristic peaks at 1617, 1453 and 1198 cm−1 of xanthene and benzene rings appeared in the polymer curve. All these evidences indicated that the target poly(AM-AR6GHS) was synthesized successfully according to the designed method.

The 1H NMR characteristic signals ((d(H) 5.749−5.773 and 6.490–6.513) in the 1H NMR spectra of AR6GHS in CDCl3 corresponding to the vinyl groups of the monomers disappeared completely in poly(AM-AR6GHS), and the signals corresponding to the xanthene and benzene rings of R6GHS (d(H) 6.0–8.0) appeared in the 1H NMR spectrum of poly(AM-AR6GHS). This observation confirmed that the hydrophobic functional monomer R6GHS had been successfully incorporated into the polymer. The signals at 1.3–3.5 ppm were assigned to the aliphatic CH3 and CH2 groups, and the signal at 3.7 ppm was assigned to the N–CH groups in the repeating R6GHS units [9]. This observation also confirmed that the R6GHS had been successfully incorporated into the polymer [16].

Effect of pH

Fluorophores are usually disturbed by the proton during the detection of metal ions, so the usage of their low sensitivities to the operational pH value are extremely expected. To explore the further utility of poly(AM-AR6GHS) as an ion selective fluorescent sensor for detecting Cu2+ and Hg2+ ions, the effects of proton (pH value) on the fluorescence spectrum of poly(AM-AR6GHS) were studied with a wide pH range between 2.64 and 11.18 in 1.00 mg/mL of poly(AM-AR6GHS) aqueous solution (λex = 500 nm). As shown in Fig. 1, there were no significant fluorescent intensity changes of poly(AM-AR6GHS) when the pH was in a range of 5.41 to 11.18 [59]. The following addition of acid into the poly(AM-AR6GHS) solution until the pH value reaching to 5.41, the fluorescence intensity of the poly(AM-AR6GHS) presented a dramatic increase. It could be explained by the opening of the rhodamine ring of the polymer caused by the strong protonation in the system [6]. Therefore, to obtain a low fluorescence background and an optimal condition for fluorescence measurement of poly(AM-AR6GHS) in the reaction mixture at the same time, neatral and base aqueous solution was selected in the following experiments [53, 60–65].

Fig. 1
figure 1

The variation of fluorescence intensity (λex = 500 nm) of poly(AM-AR6GHS) chemosensor (1.00 mg/mL) in aqueous solution over a pH range of 2.64 to 11.18 at room temperature

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Cu2+ and Hg2+ sensing properties

The concentration effects of Cu2+ or Hg2+ ions on the emission spectrum of poly(AM-AR6GHS) were also illustrated in Fig. 2a and b. As seen, while the concentration of polymer was 1.0 mg/mL and excited was at 500 nm, poly(AM-AR6GHS) exhibited weak fluorescence intensity at about 544 nm in neutral water, indicating that the spirolactam form was the predominant species. After the addition of Cu2+ or Hg2+ into the aqueous solution of poly(AM-AR6GHS), intensity increase was observed, and which was up to about 4-fold and 4.5-fold enhancement [9, 10, 17, 37, 66]. Besides, with the addition of Hg2+ into the 1.00 mg/mL of poly(AM-AR6GHS) aqueous solution, the position of the fluorescence maxima shifted progressively from 544 to 550 nm. However, no red-shifted variation was observed upon addition of Cu2+ into the 1.00 mg/mL of poly(AM-AR6GHS) aqueous solution under the same condition [10, 12]. The visible color changes occurred from colorless to pale pink or pink under visible light and from dark to greenish yellow or orange yellow under UV light at 365 nm for Cu2+ or Hg2+, respectively. The higher concentration of the Cu2+ or Hg2+ induced deeper and more distinguishble colors as shown in Fig. 2 (Inset) indicating that the Cu2+ and Hg2+ leaded to a highly conjugated rhodamine system via formation of opened-spirolactam to give a strong fluorescence emission [16]. The possible mechanism was briefly described as shown in Scheme 2. The AR6GHS units of poly(AM-AR6GHS) containing ester (−COO-), Shiff base (−CH = N-), lactam and diazanyl group could selectively chelate with metal ions by using N of imino and O atoms in structure to the form more stable pentabasic or hexabasic rings [2, 6, 7, 17, 19, 21–23, 30, 33–35, 48, 53]. The spirolactam ring-closed form could been transformed to ring-opened amide form with highly fluorescent caused by conjugation enhancement [2, 29, 30].

Fig. 2
figure 2

Fluorescence spectral changes of poly(AM-AR6GHS) (1.00 mg/mL) with different concentrations of Cu2+ or Hg2+ in aqueous solution (λex = 500 nm). The arrow indicates the trend of increasing Cu2+ or Hg2+ concentration ((a) [Cu2+] = 0-6.25 × 10−4 mol/L, (b) [Hg2+] = 0-3.25 × 10−4 mol/L). Inset: photographs recorded under visible light (Top) and UV light at 365 nm (Bottom) for aqueous solutions of poly(AM-AR6GHS) (1.00 mg/mL) in the presence of different concentrations of Cu2+ or Hg2+ (0, 0.5 × 10−4, 1.5 × 10−4 and 2.5 × 10−4 mol/L)

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Figure 3a and b were the relative fluorescence intensities of poly(AM-AR6GHS) plotted vs. Cu2+ or Hg2+ concentration. Insets of Fig. 3 were linearized responses of poly(AM-AR6GHS) to Cu2+ or Hg2+ concentration. As shown in the insets of Fig. 3a and b, the linear spectrofluorometric responses for Cu2+ or Hg2+ concentration in the range of 2.50 × 10−5 to 2.25 × 10−4 or 0 to 7.50 × 10−5 mol/L and could be expressed by the following Eqs. (1) and (2) of the calibration lines with a very good regression coefficient:

Fig. 3
figure 3

The relative fluorescence intensities as a function of Cu2+ or Hg2+ concentration. Inset: linearized responses of poly(AM-AR6GHS) to Cu2+ or Hg2+ concentration

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$$ I/{I}_0=1.461+8.826\times {10}^3\left[C{u}^{2+}\right]\kern3em \left(R = 0.9941\right) $$
(1)
$$ I/{I}_0=1.0093+5.018\times {10}^3\left[H{g}^{2+}\right]\kern3em \left(R = 0.9962\right) $$
(2)

Here I was the fluorescence emission intensity of poly(AM-AR6GHS) actually measured at a given metal cation concentration, I0 was the fluorescence emission intensity of the free poly(AM-AR6GHS), [Cu2+] and [Hg2+] represented the concentration of [Cu2+] and Hg2+ added. These linear responses were further used for detection of Cu2+ or Hg2+ concentration in the following steps through the obtained equation in a certain concentration range [11, 16–18, 38, 66–68]. The corresponding detection limits (3S/ρ: S, standard deviation; ρ, slope) were calculated to be 8.81 × 10−7 and 5.05 × 10−8 mol/L for Cu2+ and Hg2+, respectively.

Selectivity studies

The influence of various metal cations on the fluorescence intensity of the poly(AM-AR6GHS) were investigated in aqueous solution (1.00 mg/mL), which were presented in Fig. 4. Obviously, there were no significant signal response upon addition of Ba2+, Cd2+, Co2+, Fe2+, K+, Mg2+, Ni2+, Zn2+, Cr3+ and Fe3+ into the aqueous solution of poly(AM-AR6GHS). It was caused by the poor ability of the receptors with these metal cations. Besides, small fluorescence enhancement was observed for the polymer solution sample containing Ag+ and Pb2+, respectively. Only Cu2+ and Hg2+ ions exhibited the most prominent fluorescence enhancement [8, 9, 11–13, 38, 68]. Therefore, we found that poly(AM-AR6GHS) had highly selective for Cu2+ and Hg2+ ions. It could be explained by the cooperating influences of a suitable coordination geometry, the proper radius and charge density of the Cu2+ and Hg2+ ions, and the amide deprotonation ability of Cu2+ and Hg2+ ions [16, 67]. It had been found that the coordination with the R6GH unit of tridentate ligand resulted in fluorescence enhancement, while the “turn–on” switching mechanism was accomplished [18].

Fig. 4
figure 4

Fluorescence intensity changes of poly(AM-AR6GHS) (1.00 mg/mL) in aqueous solution in the presence of different metal cations (Ag+, Ba2+, Cd2+, Co2+, Cu2+, Fe2+, Hg2+, K+, Mg2+, Ni2+, Pb2+, Zn2+, Cr3+ and Fe3+) (c = 2.5 × 10−4 mol/L, λex = 500 nm)

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When the concentration of poly(AM-AR6GHS) was 1.00 mg/mL, upon addition of 2.5 × 10−4 mol/L of metal cations, the color of the solution of poly(AM-AR6GHS) changes occurred only for Cu2+ and Hg2+ ions both under visible light and UV light at 365 nm, while the addition of other ions, such as Ag+, Ba2+, Cd2+, Co2+, Cu2+, Fe2+, K+, Mg2+, Ni2+, Pb2+, Zn2+, Cr3+ and Fe3+, the color of poly(AM-AR6GHS) had any changes (Fig. 5) [16].

Fig. 5
figure 5

Photographs recorded under visible light (a) and UV light at 365 nm (b) for aqueous solutions of poly(AM-AR6GHS) (1.0 mg/mL) in the presence of different metal ions (2.5 × 10−4 mol/L)

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The competition experiments were also carried out by adding Cu2+ or Hg2+ ions with a concentration of 2.5 × 10−4 mol/L to the solution of poly(AM-AR6GHS) (1.00 mg/mL) in the presence of metal ions incluing Ag+, Ba2+, Cd2+, Co2+, Cu2+, Fe2+, Hg2+, K+, Mg2+, Ni2+, Pb2+, Zn2+, Cr3+ and Fe3+ with a concentration of 2.5 × 10−4 mol/L, respectively. The results were shown in Fig. 6, which showed that the sensing of Cu2+ or Hg2+ by poly(AM-AR6GHS) were hardly affected by these common interfering ions [37]. Selectivity and competition experiments showed that poly(AM-AR6GHS) had a remarkable selectivity toward Cu2+ or Hg2+[11, 16].

Fig. 6
figure 6

Fluorescence enhancement response of poly(AM-AR6GHS) (1.00 mg/mL) in aqueous solution to 2.5 × 10−4 mol/L of different metal ions (the red bar portion) and to the mixture of 2.5 × 10−4 mol/L different metal ions with 2.5 × 10−4 mol/L of Cu2+ (the green bar portion) or Hg2+ ions (the blue bar portion)

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Conclusion

A fluorophore monomer bearing rhodamine group (AR6GHS) was synthesized according to the designed fashion. Then, a water soluble copolymer was prepared successfully through radical copolymerization of fluorophore monomer and AM. The fluorescent character of the poly(AM-AR6GHS) was studied in this report as follows. By means of fluorescence spectroscopy, the sensibility for protons and metal cations in aqueous solution was carried out in detail. The selectiveness of poly(AM-AR6GHS) to different metal cations (Ag+, Ba2+, Cd2+, Co2+, Cu2+, Fe2+, Hg2+, K+, Mg2+, Ni2+, Pb2+, Zn2+, Cr3+ and Fe3+) were also determined. The results suggested that the highest enhancing effect was observed in the presence of Cu2+ and Hg2+ ions. In addition, the poly(AM-AR6GHS) largely preserved excellent linear response between relative fluorescence intensity and concentration in the range of Cu2+ or Hg2+ ions. On the basis of the results obtained, it can be assumed that the poly(AM-AR6GHS) was an efficient “off-on” switcher for Cu2+ and Hg2+ ions at pH between 5.41 and 11.18 and which could be a potential sensor for Cu2+ or Hg2+ ions in aqueous environment. In particular, it represents one of the few fluorescent sensors that allow a selective and sensitive detection of Cu2+ or Hg2+ in aqueous solution, with no organic co-solvent required.