Ratiometric fluorescent detection and removal of cadmium ions from aqueous solution using Indole functionalized Polysulfone

 

R. Parkavi, N. Kavitha J. Lekha, K. Dinakaran*

Department of Chemistry, Thiruvalluvar University, Vellore – 632115, India.

 

ABSTRACT:

A highly sensitive ratiometric fluorescent assay for the detection of Cd²⁺in water using metal coordinating polymer namely Indole functionalized polysulfone has been developed. Indole molecule has been introduced into the commercially available polysulfone through formation of imine linkage between indole-3-carboxaldehyde and aminated polysulfone. The indole functionalized polysulfone exhibited a fluorescent emission with a peak maximum at 391nm and 420nm, the intensity of the emission at 391nm is increased proportional to the addition of Cd²⁺ ions. The lowest detection limit for the Cd²⁺ has been found to be 2.4 nm. Further, the adsorbent capacity of the indole functionalized polysulfone has been evaluated at various pH. It is observed from the results that the adsorbent effectively adsorbs Cd²⁺at basic pH, and capable of removing 0.179g of Cd²⁺ / g of polymer.

 

 

 

INTRODUCTION:

Cadmium, a toxic heavy metal, can cause serious health problems such as kidney failure, brain damage, and DNA damage [1-2]. The cadmium levels in drinking water may be allowed upto a maximum concentration of 10 nm, as recommended by World Health Organization. Therefore, it is important to detect cadmium at concentrations around this limit with high sensitivity and selectivity. There have been many studies on the detection of Cd²⁺ ions by utilizing electrochemistry or fluorescence [3-9]. Fluorescence sensors for cadmium detection are reported in the literature based on small organic molecules and fluorescent polymer-based chemosensors.

 

Cheng et al⁴ reported a selective and sensitive OFF-ON fluorescent sensor based on boron containing organic molecule, employing the PET mechanism with a detection limit of 0−1μm toward Cd²⁺. A similar fluorescent sensor based on acetamidoquinoline demonstrating high selectivity for sensing Cd²⁺ with and picomolar sensitivity (K_d = 0.25±0.03 pM) was reported by Xue et al⁵ and Zhou et al.⁶Pyridyl ligands are also employed to detect cadmium ion using emission spectrometry and has good selectivity for Cd²⁺ over other metal ions. The change in fluorescence intensity due to intramolecular charge-transfer mechanism has been utilized to make a optical probe for Cd²⁺ detection [7-8].

 

Polymer based fluorescence sensors demonstrated significance advantage over small organic molecules mainly in signal amplification is an implicit advantage. [9] For instance, Poly(thiophene-3-yl-acetic acid 8-quinolinyl ester) has been employed [10] as a fluorescent material for the detection of cadmium. The fluorescence of Poly(thiophene-3-yl-acetic acid 8-quinolinyl ester) polymer is quenched by copper, cadmium and lead metal ions and quenching is sensitive to concentrations of metal ions due to FRET [11-13]. Bunz and coworkers reported that aqueous soluble polyphenylene-acetylene polymer as a potent sensing platform for lead salts in the aqueous solution [14]. However, most of these sensors are intensity based and do not provide sufficient accuracy for quantitative measurements because of their emission intensity is known to be conditioned by many variables, such as the sample environment, sensor concentration, bleaching, and instrumental efficiency. In contrast, ratiometric sensors can eliminate most or all ambiguities by built-in calibration of the two emission bands [15-18].

 

Ratiometric signal output for sensing Cd²⁺ based on the fluorophore 4-isobutoxy-6-(dimethylamino)-8-methoxyquinaldine has been reported [19]. Lin et al²⁰ developed a ratiometric fluorescent sensor based on a derivative of 4,5-diamino-1,8-naphthalimide which was successfully differentiated Cd²⁺ and Zn²⁺ by observing internal charge transfer processes in the metal legend complex. The selectivity of optical sensor based on 4,5-diamino-1,8-naphthalimide as the fluorophore to Cd²⁺ over some other metals was also observed. In this manuscript, we report the ratiometirc fluorescent response of Cd²⁺ through its coordination interaction with indole functionalised polysulfone. Wang et al developed ratiometric fluorescent sensor using a fluorescent molecule such as benzimidazole derivative for the detection Cr³⁺ and Fe³⁺ in water sample. The sensor showed a lowest detection limits for Cr³⁺ and Fe³⁺ as 25 μm and 2μm, respectively [21]. A ratiometric sensor for detecting Zn²⁺ with a sensitivity limit of detection of 33.6nm was reported by Ma et al. [22]

 

In the present research, we have developed sensitive ratiometric fluorescence sensor based on Indole functionalized polysulfone for the detection of cadmium present in aqueous solution. Indole moiety has been introduced into the commercially available polysulfone through formation of imine linkage between indole-3-carboxaldehyde and aminated polysulfone. The photoluminescence peak at 375nm increases proportional to Cd²⁺ concentration while the other emission band at 420nm remains unaffected, leading to ratiometric response, due to the intramolecular charge transfer mechanism. [4] Thus, the indole functionalized polysulfone has been utilized to quantify the cadmium present in environmental water samples, In addition, cadmium removal studies from aqueous solution was also studied using indole functionalized polysulfone as adsorbent.

 

EXPRIMENTAL SECTION:

Materials:

Reagent grade of Polysulfone and Indole-3-aldehyde are purchased from Alfa –Acer, Nitric acid, Chloroform, Stannous chloride, Acetic acid, O –Chlorobenzoic acid were purchased from Avera. Infrared spectra are recorded on a Perkin–Elmer 1725X FTIR Spectrometer. NMR spectra are recorded in on Bruker instrument using DMSO d₆ as solvent. Photoluminescence spectra are recorded in a Perkin Elmer Fluorescence spectrometer LS45.

 

Nitration of polysulfones (Psf):

About 1g of PSf was dissolved in 25mL of CHCl₃ at 25 °C. Then, 5 mL of a mixture of Nitric acid and sulfuric acid were added drop – wise at 25°C. The volume ratio of nitric acid and sulfuric acid is 4:1. The reaction medium was allowed to stir for 30 min. The formed yellow colour nitrated PSf (PSfNO₂) was precipitated from methanol and purified.

 

Reduction of nitrated polysulfones (PS –NH₂):

About 0.5g of Sf –NO₂ was dissolved in 15mL of CHCl₃ in a two – necked flask equipped with a reflux condenser. Then, a solution of 5g of SnCl₂, 2H₂O and 0.16g of KI in 12mL of HCl – glacial acetic acid mixture (2:1) was added in dropwise at 60°C under stirring. The resulting mixture was further refluxed for 3 hours and then cooled to room temperature. The orange coloured solid polymer obtained was filtered and dried at 40°C under vacuum.

 

Synthesis of Indole functionalized polysulfone:

Aminated polysulfone 1g was dissolved in 25mL of ethanol and Indole-3-carboxaldehyde (0.3gram) was dissolved in 25mL of ethanol. The mixture was refluxed for 6hours at 60°C then cooled to room temperature. Then the solution was poured into acetone to obtain the precipitate. The precipitate was washed with water and filtered.

 

Removal of Cd²⁺ ion from 0.01M Cd (CH₃COO)₂ Solution:

0.01M of aqueous Cd (CH₃COO)₂solution was prepared and then adsorption studies were carried out in various solution pH using the prepared Indole pendant polysulfone as adsorbent. The concentration of solution was monitored in the various pH such as 3, 7, 10 and the concentration of unadsorbed Cd2+ is estimated using Atomic absorption spectrometer.

 

RESULTS AND DISCUSSIONS:

 

Scheme – 1: Synthesis of indole functionalized polysulfone

 

Nitration of polysulfone has been carried out using nitrating mixture of nitric and sulfuric acid at room temperature as shown in Scheme-1. The yellow colour product obtained was purified by repeated precipitation method and it has been characterized by FTIR spectroscopy and presented in Figure 1. The FTIR spectra of the Nitrated polysulfone exhibited characteristic absorption bands are at ν _SO2 = 688 cm^-1 ; ν _C-O-C = 1238 cm^-1 ; ν_C-N = 2036 cm^-1 ; ν_C-H = 2968 cm^-1(symmetric stretching ) ; ν_C-H = 2924 cm^-1 (asymmetric stretching ) ; ν_C-H = 1484 cm^-1 (bending vibration) ν_NO2 = 1586 cm^-1 (sharp peak) ; ν_NO2 = 1321 cm^-1 ( broad peak ) it will conform to a nitrated polysulfone.

 

Figure 1: FTIR - Spectrum of Nitrated polysulfone

 

Aminated polysulfone has been obtained by the reduction of nitrated polysulfone using stannous chloride, potassium iodide and HCl – glacial acetic acid mixture under reflux for 3 hours, then cooled room temperature. The orange colour polymer product obtained was purified by repeated precipitation method and it has been characterized by FTIR spectroscopy and shown in Figure 2 The characteristic absorption bands (FTIR) are ν _O-H =3430 cm^-1, ν_SO3H = 1170 cm^-1 (asymmetric stretching) ν_SO3H = 1028 cm^-1 (symmetric stretching). ν _{N – H} = 3100 cm^-1 (symmetric stretching); ν_{C – C} = 1672 cm^-1, ν_{N – H} = 1620 cm^-1, bending vibration which confirms the presence of amine group in the polysulfone. Indole functionalized polysulfone has been synthesized by refluxing aminated polysulfone and indole-3-carboxaldehyde in ethanol for 6 hours. The white colour product obtained was purified by repeated precipitation and has been characterized by FTIR spectroscopy.

 

The Figure 3. Illustrates the UV - Visible spectra of polysulfone. The UV – Visible absorption of spectrum of polymer shows a broad peak at 264 nm owing to π – π*.

 

Figure 2: FTIR spectrum of aminated polysulfone

 

Figure 3: UV - Visible spectrum of polysulfone

 

Figure 4: UV - Visible spectrum of Indole functionalized polysulfone

 

The Figure 4 presents the UV - Visible absorption spectra of Indole functionalized polysulfone. The UV – Visible absorption of spectrum shows a broad peak at 266 nm owing to π – π* in polysulfone and a absorption peaka at 362 nm and 395 nm (owing to n –π* transition) are corresponding to absorption of Indole moiety.

 

Figure 5. FTIR- Spectrum of Indole functionalized polysulfone

 

The FTIR of the Indole functionalized polysulfone is shown in Figure 3. The characteristic absorption bands are, ν_C-H = 3113 cm^-1, ν _SO2= 700 cm^-1, ν_C-C = 1076 cm^-1, ν_C=N = 1386 cm^-1.

 

Figure 6. (a) Photoluminescence spectra of Indole functionalized polysulfone with various concentration of Cadmium ions. (b) Ratio of intensities (I₁ and I₂) with respect to Cadmium ion concentration

 

The Figure 6 shows the PL spectrum of the Indole functionalized polysulfone. The emission spectra showed two emission peaks, with a strong fluorescence maximum at λ _max = 391nm another one at λ _max = 420 nm. The addition of cadmium solution into the Indole functionalized polysulfone resulted in increase of the λ _max = 391nm florescence intensity. Thus, confirms the presence of Cd²⁺ in the aqueous solution. Further, the increase in fluorescence intensity was found to be proportional to the concentration of added Cd²⁺ ions, thus forms the basis for quantitative estimation of Cd. In the present study the limit of lowest detection of cadmium was found to be 2.4nM

 

Cadmium ion removal from aqueous solution:

Table 1: Cadmium adsorption using Polysulfone and indole functionalized polysulfone

S. No	Ph	Strength of Cadmium solution after filtration (initial strength: 0.01M)	Amount of Cadmium Removed
1.	3	0.0098M	0.0096 g
2.	7	0.0081M	0.092 g
3.	10	0.0062M	0.1796 g

 

The adsorbent capacity of the indole functionalized polysulfone has been evaluated at various pH range and presented in Table – 1. The adsorbent can be removed from effluent by filtration, separation and can be recycled and reused. The effect of pH on adsorption of Cd²⁺ on Indole functionalized polysulfone is given in Table 1 and Figure 7. It is observed from the data that the Indole functionalized polysulfone effectively adsorbs Cd²⁺at basic pH. However, it was found that the percentage removal of metal ions is not changed by increasing initial concentration of Cd²⁺, this is may be due to fact that the Cd²⁺ was removed by forming complex with adsorbent²³ as shown in Scheme-2.

 

Figure 7: Cadmium absorption by Indole functionalized Polysulfone.

 

Scheme-2. Cadmium coordination in indole functionalized polysulfone

 

CONCLUSIONS:

In summary, we have developed a simple mix and detect fluorescence assay for rapid detection of Cd²⁺ by Indole functionalized polysulfone. In this work, highly sensitive fluorescent Sensor based on the Indole functionalized polysulfone towards the detection of cadmium (II) was successively developed. The sensitivity and selectivity of the proposed sensor was evaluated that the increase in concentration of Cd²⁺results in gradual enhancement of fluorescence intensity. The detection limit was found to be 2.4nM thus, the developed fluorescent sensor might pave the way for the detection of cadmium ions in the environmental samples. The developed protocol assay is convenient and cost effective. In addition, the Indole functionalized polysulfone can also be used as adsorbent to remove cadmium from water samples.

 

ACKNOWLEDGEMENT:

The author K. Dinakaran acknowledges the financial support of SERB, Department of Science and Technology, New Delhi, India, through Grant No. EEQ/2016/000049.

 

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Received on 05.05.2020                    Modified on 22.05.2020

Accepted on 01.06.2020                   ©AJRC All right reserved