Solid-State Fluorescent Organic Polymers for Visual Detection and Elimination of Heavy Metals in Water

Abstract

Selective sensing and removal of toxic heavy metals from water are highly essential since their presence poses significant health and environmental hazards. Herein, we designed and synthesized a novel fluorescent nonconjugated organic polymer by strategically incorporating two key functional groups, namely, a dansyl fluorophore and dithiocarbamate (DTC). Different characterization techniques, including ¹H nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), Fourier transform infrared (FTIR), and fluorescence spectroscopy, were performed to understand its structure and material properties. The quantum yield of 4.72% and its solid-state fluorescence indicate that it has potential for various applications in several technological and scientific domains. In this study, we investigated a specific application involving the detection and elimination of heavy metals from water. Interestingly, the presence of dansyl and DTC moieties demonstrated remarkable selectivity toward Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, and Fe²⁺ sensing, displaying distinct color changes specific to each metal. Cu²⁺ resulted in a yellow color, Co²⁺ showed a green color, Ni²⁺ displayed a pale yellowish-green color, and Fe²⁺/Fe³⁺ exhibited a brown color. The LOD (limit of detection) for each metal was obtained in the nanomolar range by using a fluorescence spectrometer and the micromolar range from UV–visible spectra: 13.27 nM and 0.518 μM for Cu²⁺, 8.27 nM and 0.581 μM for Co²⁺, 14.36 nM and 0.140 μM for Ni²⁺, 14.95 nM and 0.174 μM for Fe²⁺, and 15.54 nM and 0.33 μM for Fe³⁺. Moreover, the DTC functionality on its backbone facilitates effective interaction with the aforementioned heavy metals, subsequently removing them from water (except Fe²⁺ and Fe³⁺), validating its dual functionality as both an indicator and a purifier for heavy metals in water. The polymer exhibited a maximum adsorption capacity of 192.30 mg/g for Cu²⁺, 159.74 mg/g for Co²⁺, and 181.81 mg/g for Ni²⁺. Furthermore, this approach exhibits versatility in crafting fluorescent polymers with adjustable attributes that are suitable for a wide range of applications.

Introduction

A staggering reality unfolds as the World Health Organization reveals that more than 785 million people lack adequate access to drinking water, while an alarming 884 million are denied the basic right to safe and secure drinking water.¹ According to a recently released United Nations report, our planet is estimated to face the imminent threat of a 40% water shortage within the next 10 years.² As per the data provided by NITI Ayog, approximately 600 million individuals in India are at risk of experiencing severe water stress, resulting in an ongoing water crisis.³ To exacerbate the situation, recent findings have revealed that every minute, six infants in developing countries succumb to mortality due to unsafe water conditions, underscoring the critical need for improved water safety measures.⁴ An upsurge in energy production and an exponential increase in the usage of heavy metals in industrial processes have enabled these metals to find their way to water bodies, drastically affecting marine life.⁵ The presence of heavy metals like iron, cobalt, nickel, and copper in water beyond safe consumption levels can impose serious health threats such as cancers, birth defects, multiple organ dysfunctions, and so on.⁶ This highlights the pressing global concern of water contamination, which demands immediate attention and resolution.

The traditional methods employed in heavy metal remediation, such as chemical precipitation, the utilization of sorbents and membranes, electrolysis, and ion-exchange techniques, have been progressively replaced by adsorptive methods due to their inherent challenges, which encompass increased economic and energy requirements, reduced removal efficiency, complex regeneration processes, fouling issues, and the generation of significant sludge volumes.⁷⁻¹¹ Over time, metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) have gained attraction as promising options for the capture of heavy metals. This is attributed to their enhanced recyclability, selectiveness, straightforward modification, and efficient adsorption capabilities.^12,13 However, MOFs have demonstrated moderate stability, whereas COFs have comparatively limited potential for modification.¹⁴ Organic polymers with characteristics such as high removal efficiency, facile chemical tunability, stability, controllable surface area, and pore size are more likely to be utilized in environmental settings. Organic polymers are used to remove a variety of analytes, including organic dyes, volatile organic chemicals, gases, hazardous metal ions, and many more.¹⁵⁻²⁰ At present, numerous organic polymers are being developed that have applications in both analyte detection and removal.²¹⁻²³ However, their synthesis requires a sophisticated instrument, a multistep process, a higher reaction temperature, or an inert reaction environment. The aforementioned circumstances present obstacles to their practical implementation. This compelled us to develop a tunable organic polymer with versatile applications, economical steps, and benign reaction conditions. The fabrication of a bifunctional organic polymer that acts as a sensor and eliminator of various heavy metals from water constitutes a significant advancement, integrating cost-effectiveness, sustainability, efficiency, and efficacy into a singularly compelling solution.

This report presents a novel dansyl-tagged dithiocarbamate (DTC) linked fluorescent organic polymer with the ability to serve as a cost-effective and environmentally friendly option for both selective visual sensing and removal of toxic heavy metals from water. Moreover, the synthetic procedure for the polymer does not require any harsh conditions, and all of the synthetic stages are straightforward, occurring at room temperature and in air. Furthermore, the developed dansyl-appended dithiocarbamate (DTC)-based organic polymer (D-DTC-OP) helps in visual detection (Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺ and Fe³⁺) and removal of heavy metal ions like Cu²⁺, Co²⁺, and Ni²⁺ from water. Dansyl chloride was chosen to be the fluorophore due to its robust optical characteristics that encompass near UV absorption, fluorescence in the visible region with a substantial Stokes shift, and the ability of its emission spectra to respond sensitively to chemical surroundings.²⁴ Various studies have been conducted on the detection of different metal analytes, as well as the removal of individual analytes by a dansyl-functionalized polymer.²⁵⁻²⁷ However, this research introduces a novel approach by developing an organic polymer linked to dansyl-tagged dithiocarbamate. This polymer has the ability to detect and simultaneously remove multiple metal ions from water. Moreover, the quantification of metal ions in the solution can be done by using the same polymer, which is an interesting process because it allows us to quantify the metal ions in water without using very sophisticated instruments such as atomic absorption spectroscopy (AAS), X-ray absorption spectrometry (XAS), surface-enhanced Raman scattering (SERS), inductively coupled plasma atomic, emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and online coupled systems (e.g., gas chromatography or liquid chromatography coupled with AAS, AFS, or ICP-MS). The DTC functional group has also been tailored to its backbone as these organosulfur domains are acknowledged for their capacity to interact with heavy metal ions. This interaction is based on the principle of a soft–soft interaction between sulfur and heavy metal ions. This characteristic is then harnessed for the elimination of hazardous heavy metals from polluted water sources. DTCs are a notable category of substances that play important roles in diverse domains, such as pharmaceuticals, chemical synthesis, and as amino-protecting agents and linkers in peptide chemistry.²⁸⁻³¹ The occurrence of DTC functional groups in a wide array of natural compounds exhibiting anticancer, antifungal, herbicidal, and pesticidal activities underscores their wide-ranging potential in diverse fields of application.³²⁻³⁶ Hence, through the integration of these dual functionalities, we have developed an innovative organic polymer that serves a dual purpose. It not only serves as a visual indicator by exhibiting a distinct color change in the presence of different hazardous metals in water (Cu²⁺—yellow, Co²⁺—green, Ni²⁺—pale yellowish-green, Fe²⁺/Fe³⁺—brown) but also excels in removing them from contaminated water (except Fe²⁺/Fe³⁺). Additionally, the removal studies were conducted using various concentrations and fitted to the Langmuir isotherm equation to determine the polymer’s maximum uptake capacity (Q_m). The Q_m values for Cu²⁺, Co²⁺, and Ni²⁺ were 192.30, 159.74, and 181.81 mg/g, respectively. Furthermore, a study was conducted on the reusability of the polymer, which showed that the polymer maintains its efficiency at a level of 80% throughout four cycles.

Experimental Methods

Materials Used

Dansyl chloride was procured from BLD Pharma; diethanol amine from SRL chemicals; chloroacetyl chloride, carbon disulfide, polyethylene glycol-200 (PEG-200), and dimethyl sulfoxide (DMSO) from Spectrochem; N,Ń-dibenzyl ethylene diamine from Sigma-Aldrich; and dichloromethane, ethyl acetate, and hexane from Finar. The metal (Cr³⁺, K⁺, Al³⁺, Na⁺, Co²⁺, Fe³⁺, Hg²⁺, Fe²⁺, Mg²⁺, Pb²⁺, Ca²⁺, Ni²⁺, Mn²⁺ and Cu²⁺) salts were brought from Nice and Merck. The syringe filter (filtering rate: 0.2 μm, diameter:13 mm) and the disposal syringe were brought from Cole-Palmer.

Instrumentation

LC-MS analysis was conducted using a Shimadzu LC-MS-8045 instrument equipped with a Sprite TARGA C18 column (40 mm × 2.1 mm, 5 μm). Mass detection was performed at 210 and 254 nm in positive mode. The mobile phase for LC-MS consisted of water with 0.1% acetic acid (solvent A) and acetonitrile with 0.1% acetic acid (solvent B). ¹H nuclear magnetic resonance (NMR) spectra were acquired using a Bruker AV III 500 MHz spectrometer, and the data were analyzed using MestReNova software (version 8.1.1). The chemical shifts in the ¹H NMR spectra are expressed in units of parts per million relative to tetramethyl silane (TMS). The absorbance measurements were performed using a Biotek Epoch 2 microplate reader equipped with 96-well plates. The fluorescence measurements were performed using a PerkinElmer FL 6500 instrument. All fluorescence spectra were collected at a temperature of 25 °C, with an excitation wavelength of 345 nm and excitation/emission slit widths set to 10 nm. FTIR spectroscopy was conducted using a Shimadzu IR Tracer 100 instrument, employing the KBr pellet method. The obtained spectra were visualized and plotted using OriginPro 8.5.1. X-ray diffraction investigations were performed using a Rigaku XRD Smart lab, a 9 kW system. For this, radiation (Kα) of approximately 1.54 Å (Ω) was used. Scanning electron microscopy (SEM) images were captured using a Carl Zeiss Gemini SEM 300 instrument, which offered a magnification of 2,000,000×. Thermal gravimetry analysis was performed using a PerkinElmer Inc. Thermal Analyzer STA 8000. The elemental analysis was carried out by a Thermo Scientific ICAP RQ ICP-MS analytical instrument. The helium KED mode (kinetic energy discrimination) analysis mode was followed for the ICP-MS experiment. The minimum detection limit for the ICP-MS instrument was 0.1 ppb.

General Synthesis

The polymer synthesis was initiated with the reaction between dansyl chloride and diethanol amine in the presence of triethylamine (TEA) as a base and dichloromethane (DCM) as a solvent. The resulting product was then treated with chloroacetyl chloride in a DMF solvent to yield the chloroacetylated product. The progress of the reaction was observed using TLC in a hexane and ethyl acetate mixture (7:3 ratio) and visualized under UV light. Upon completion of the reaction, any excess chloroacetyl chloride was neutralized by adding a sodium bicarbonate solution until the evolution of CO₂ ceased. The reaction mixture was extracted with ethyl acetate and water, and the organic layer was filtered through anhydrous Na₂SO₄. After removal of the solvent under low pressure, the product was obtained with high purity. Subsequently, this product was then subjected to a 12 h treatment with N,N-dibenzyl ethylene diamine in the presence of solvent PEG-200 and carbon disulfide. The resulting fluorescent solid underwent multiple washes with acetone and water to eliminate impurities. The solid was subsequently dried and stored at room temperature.

Procedure for Fluorescence Analysis

A stock solution of the polymer, D-DTC-OP, was prepared by adding 15 mg in 1 mL of DMSO. The metal-ion solutions of Cr³⁺, K⁺, Al³⁺, Na⁺, Co²⁺, Fe³⁺, Hg²⁺, Fe²⁺, Mg²⁺, Pb²⁺, Ca²⁺, Ni²⁺, Mn²⁺, and Cu²⁺ were prepared at 5 mM in water, and the required amounts were pipetted out into the cuvette for absorption and fluorescence analyses. The maximum excitation wavelength was observed at 340 nm, while the emission wavelength was recorded at 507 nm, leading to a significant Stokes shift of 167 nm.

Methods for UV–Visible Sensing Studies

First, a polymer stock solution was prepared by dissolving 3 mg of the polymer in 1 mL of a dimethyl sulfoxide (DMSO) solvent. Afterward, an equal volume of the polymer stock solution was added to each well of the 96-well plate, followed by the addition of different concentrations of metal-ion solutions. A noticeable sequence of color variations was observed as the concentration of metal was increased, ranging from lower to higher. To ascertain the absorbance value, UV–visible absorbance spectra were also recorded.

Limit of Detection

The limit of detection (LOD) for all of the metals was determined using the following equations.

where “σ” represents the standard deviation for the sensor and “K” denotes the binding constant obtained from the quenching study.

Procedure for the Metal Removal Experiment

First, the polymer (D-DTC-OP) was dispersed in water. Then, it was treated with the metal solution and stirred for 5 min. Following the treatment, the polymer was separated using filtration, and the remaining supernatant water was analyzed using fluorescence spectrometry employing our polymer as a sensor to determine the presence or absence of metal ions.

Quantification of Metal Ions

The quantification of metal ions was determined by using fluorescence spectroscopy. Initially, the fluorescence of a polymer solution was measured. Then, a known concentration of metal ions was added to the polymer solution, causing a decrease in the fluorescence emission spectra. The difference between the maximum fluorescence intensity observed by the polymer (I₀) and the maximum fluorescence intensity of the polymer after adding the metal solution (I) was considered as a 100% metal-ion concentration. Similarly, the same experiment was repeated by titrating the polymer with the supernatant solution obtained after the removal. The difference in fluorescence intensity (I₀ – I) obtained after the removal of the metal ions was compared with the initial difference, and the percentage of removed ions was calculated. The equation for calculating the percentage of removal is as follows

Uptake Capacity

One milligram of the polymer (D-DTC-OP) was added to the metal solution (1 mL) in the range of the concentration (in ppm) and stirred for 1 h. Next, the solution was filtered, and the supernatant was collected. The percentage of remaining metal ions in the supernatant liquid was monitored by the synthesized polymer sensor itself using fluorescence spectroscopy. The collected data were fitted into Langmuir adsorption isotherm models (linear and nonlinear) using the following equation

where C_e is the final equilibrium concentration of the solution, Q_e is the uptake capacity at the point, Q_m is the maximum uptake capacity, and K_L is the Langmuir constant.

Reusability Experiment

At first, the polymer was evenly dispersed in water and subjected to treatment with a metal solution. Subsequently, the solution was stirred for 1 h. Following this, the polymer solution was centrifuged to separate the pellet (settled polymer) from the residual liquid (supernatant water). Further, the latter was analyzed using fluorescence spectroscopy with the synthesized polymer sensor. This analysis was performed to determine the presence or absence of metal ions. Subsequently, the pellet was subjected to a washing process using hydrochloric acid (0.1 mol/L), followed by washing with water until the pH of the polymer solution became neutral. The washed polymer was then used for the next removal cycle, and the same procedure was carried out three times.

pH-Dependent Removal

For pH-dependent adsorption, first the metal solution was made in the acidic, basic, and neutral solutions, followed by the same amount of the polymer being treated in the respective pH solution. After that, the solution was stirred for 1 h and the solution was filtered through the syringe filter to remove the polymer. The filtered solution was titrated with the sensor, and based on fluorescence quenching, the amount of metal ions was quantified.

Results and Discussion

The ongoing demand for affordable and easily synthesized materials with desired properties facilitated us to fabricate an organic polymer via a three-step synthetic strategy utilizing inexpensive and easily accessible reagents. The reaction started with the fluorophore, dansyl chloride reacting with diethanol amine (Scheme 1) in the presence of triethylamine (TEA) as a base and dichloromethane (DCM) as a solvent to form N-dansyl diethanol amine (Scheme 1) with terminal hydroxy groups. In the subsequent step, the hydroxy groups were subjected to functionalization through a reaction with chloroacetyl chloride, yielding the chloroacetyl derivative (Scheme 1). After each synthetic step, the produced compounds were analyzed using HPLC trace, mass spectrometry, and ¹H NMR (Figures S1–S6). Following this, the chloroacetyl derivative was subjected to a three-component polymerization reaction with N,N′-dibenzyl ethylene diamine and carbon disulfide, resulting in the synthesis of the desired polymer dansyl-appended DTC-based organic polymer (D-DTC-OP as depicted in Scheme 1). Polyethylene glycol-200 (PEG-200), an ecofriendly solvent, was employed as the reaction medium, and the reaction mixture was continuously stirred for 12 h at room temperature. The resultant reaction mixture was washed with water and acetone multiple times to eliminate any impurities until a white solid product settled at its bottom. Then, it was centrifuged, filtered, dried properly, and then subjected to various characterization techniques. The polymer (white solid powder) showed solid-state fluorescence under UV light. It is worth noting that all three stages of this procedure were conducted at room temperature, employing mild reaction conditions, without the necessity of an inert atmosphere, and utilizing cost-effective starting materials.

Scheme 1. Synthetic Procedure for the Dansyl-Appended DTC-Based Organic Polymer (D-DTC-OP).

The polymer was characterized by Fourier transform infrared (FTIR) spectroscopy, ¹H nuclear magnetic resonance (NMR) spectroscopy, and energy-dispersive X-ray analysis (EDAX). ¹H NMR (Figure 1a) assured the product formation due to the presence of signals corresponding to all different types of protons in the D-DTC-OP, as proposed in its design. In FTIR spectroscopy (Figure 1b), the polymer showed characteristic peaks at 1045 cm^–1 (stretching frequency of C=S), 1190 cm^–1 (stretching frequency of C–N), 1331 cm^–1 (stretching frequency of ester C–O), and 1616 cm^–1 (stretching frequency of C=O). The powder X-ray diffraction (XRD) studies of the organic polymer also appeared to be unique (Figure 1c). In contrast to the conventional organic polymers, XRD data showed that the D-DTC-OP is crystalline in nature. This was affirmed by the presence of sharp peaks in the XRD pattern, which, in turn, implies the existence of a long-range molecular ordering and crystallinity within the polymer.

Figure 1.

(a) ¹H NMR spectra of D-DTC-OP in DMSO–D₆ (500 MHz); the solid triangle and * represent the residual proton signal for DMSO–D₆ and tetramethyl silane (TMS), respectively. (b) FTIR spectra of D-DTC-OP. (c) X-ray diffraction patterns of D-DTC-OP.

The varying dimensions of granular/chip-like fragments stalked vertically define the morphological structure of the polymer, as observed from scanning electron microscopy (SEM) images (Figure S7). Moreover, the images suggest that the polymer has a specific arrangement throughout. SEM is often coupled with energy-dispersive X-ray spectroscopy (EDS) (Figure S8) or X-ray microanalysis to determine the elemental composition of the sample. The EDAX analysis yielded the elemental ratios present in the material, thereby confirming the D-DTC-OP. Furthermore, the EDAX experiment revealed that the percentage composition closely aligns with the calculated percentage, thereby validating the polymer’s purity. This distinct structural characteristic of D-DTC-OP can be exploited in the field of electronics and optoelectronics and in the fabrication of membranes with tunable pores for water treatment and gas separation. Thermal stability was studied using TG-DTA analysis (Figure S9). The polymer was stable up to 120 °C and showed 18% decomposition at 150 °C and complete decomposition at 300 °C. The 18% mass loss observed in our case at 150 °C was caused by the carbon disulfide, carbon dioxide, and sulfur dioxide that were released from the polymer (Figure S9b).

Understanding the quantum yield of a fluorescent material is essential for researching its applications in the materials and biomedical fields. In this study, we determined the quantum yield of D-DTC-OP relative to quinine sulfate. The quantum yield of D-DTC-OP was determined to be 4.72% (Figure S10). In addition, the polymer had solvatochromism capabilities, as shown in Figures 2d and S11. As the polarity of the solvents increased from toluene (nonpolar) to DMSO (polar), a significant shift toward longer wavelengths (bathochromic shift) was observed. The maximum absorption wavelength (λ_max) increased from 497 to 531 nm, as depicted in Figure 2d.

Figure 2.

(a) Solid-state fluorescence emission spectra, (b) fluorescence emission spectra of D-DTC-P in water (1% DMSO, concentration of the DMSO stock was 10 mg/2 mL) excited at a 340 nm wavelength, (c) D-DTC-OP (in solid state) under normal light (left) and under long UV (right) and D-DTC-OP (in solution) under normal light (left) and under long UV (right), and (d) solvatochromism properties of D-DTC-OP.

Followed by the synthesis and characterization of D-DTC-OP, it was employed for environmental remediation; in this case, for the detection and removal of toxic heavy metal contaminations in water. The rationale behind designing D-DTC-OP is based on the soft–soft interaction of sulfur of the DTC group and heavy metal, leading to strong interactions with the heavy metals. The detection of heavy metals is essential due to their widespread occurrence as contaminants in water bodies. Uncontrolled levels of these heavy toxic metals can have severe adverse effects on livestock and human health. Therefore, it is crucial to develop materials that can accurately detect and remove such heavy metals from contaminated water.

D-DTC-OP is well soluble in DMSO and is soluble in water in the presence of 1% of DMSO. Fourteen different metals (Cr³⁺, K⁺, Al³⁺, Na⁺, Co²⁺, Fe³⁺, Hg²⁺, Fe²⁺, Mg²⁺, Pb²⁺, Ca²⁺, Ni²⁺, Mn²⁺ and Cu²⁺) were taken for sensing studies. For this experiment, a stock solution of a polymer (15 mg) was prepared in 1 mL of DMSO. Thereafter, 5 μL was taken from the stock solution and added to the vials having 1.5 mL of 1 mM metal solutions. Interestingly, it was found that a color change occurred only in the case of five metal ions Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Fe³⁺, whereas the solution remained unchanged for the other metals. The addition of Cu²⁺ resulted in the rapid formation of a yellow color, Co²⁺ led to the appearance of a green color, Ni²⁺ resulted in a greenish-yellow color, and the addition of Fe²⁺ and Fe³⁺ produced a brown color (Figure 3a). From this, it can be inferred that the polymer is capable of detecting the presence of Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Fe³⁺ ions, respectively, with no discernible impact on other metals. The same results were depicted in the absorbance spectra (Figure 3b). Each metal displayed a distinct absorbance peak: Co²⁺ exhibited a new peak at 584–740 nm with λ_max= 650 nm, Cu²⁺ showed a peak at 385–546 nm with λ_max= 434 nm, Ni²⁺ showed a peak at 370–418 nm with λ_max= 392 nm, and Fe³⁺/Fe²⁺ absorbed in two regions 326–477 nm with λ_max= 392 nm and 484–686 nm with λ_max= 593 nm in the absorbance spectra. Furthermore, the limit of detection was also calculated from the absorption spectra. The obtained LODs were 0.518 μM for Cu²⁺, 0.581 μM for Co²⁺, 0.140 μM for Ni²⁺, 0.174 μM for Fe²⁺, and 0.33 μM for Fe³⁺. The LOD value signifies that the polymer also has the potential to detect the metal ions in the micromolar range visually as well as from absorption spectra (Figures S14–S23). The fluorescence studies were used to understand the interaction between the sensor (D-DTC-OP) and the metal analytes. Initially, for the fluorescence study, a stock solution of the polymer was prepared by dissolving 15 mg of DTC-OP in 1 mL of DMSO. Subsequently, 10 μL of the polymer stock solution was introduced into a cuvette containing 2 mL water. The fluorescence spectra of this solution were recorded using an excitation wavelength of 340 nm. The polymer solution in the cuvette was then titrated with 10 μL of metal ions (5 mM), and fluorescence spectra were obtained. The above procedure was carried out with all of the metal ions. The spectra obtained after each addition were finally stacked together. It was found that the fluorescence was significantly quenched in the presence of Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Fe³⁺ ions. However, no noticeable alteration in fluorescence intensity was detected with the remaining nine metals (Figure 3c). The LOD (limit of detection) was obtained from fluorescence titration studies at lower concentrations. The LOD value was calculated using the equation LOD = 3σ/K, where “σ” represents the standard deviation of the sensor alone and K represents the slope of the regression curve generated from the plot of fluorescence intensity against the concentration of metal ions (Figure S24–S28). The obtained LOD values for Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Fe³⁺ are 13.28, 8.27, 14.36, 14.95, and 15.54 nM, respectively (Table 1). The sensing experiments were conducted by using paper strips as well. The paper strip was treated with D-DTC-OP, and then the corresponding metal-ion solution was applied to the treated area. The addition of metal ions resulted in the quenching of fluorescence, and also apparent color changes were observed (Figure S13).

Figure 3.

(a) Vial picture of D-DTC-OP after the addition of metal ions under normal light. (b) Absorbance spectra of D-DTC-OP upon the addition of various metal ions stacked in succession. (c) Fluorescence emission spectra of D-DTC-OP in water (0.5% DMSO, the concentration of the DMSO stock was 10 mg/2 mL D-DTC-OP) upon the addition of various metal ions (10 μL from the 5 mM metal stock solution).

Table 1. Limit of Detection for Sensing and Removal (mg/g) Efficiency of D-DTC-OP.

analytes	distinct wavelength (λ) of absorbance (nm)	limit of detection (LOD) of polymer (from fluorescence)	limit of detection (LOD) of polymer (from absorbance)	removal efficiency (mg/g)
Ni2+	370–418 (λmax: 392)	14.36 nM	0.140 μM	179.65 ± 6.79
Co2+	584–740 (λmax: 650)	8.27 nM	0.581 μM	163.31 ± 2.94
Cu2+	385–546 (λmax: 434)	13.27 nM	0.518 μM	192.30 ± 0.90
Fe2+	326–477 (λmax: 392)	14.95 nM	0.174 μM
	484–686 (λmax: 523, 610)
Fe+3	326–477 (λmax: 392)	15.54 nM	0.33 μM
	484–686 (λmax: 523, 610)

The detection of toxic metals in water is a critical step; however, remediation of the water must occur after detection in order to make the water pollutant-free. There has been a limited amount of research explored for a single material having potential in sensing as well as removal. Motivated by the pressing need, we carefully designed this material to be capable of specifically detecting and removing contaminants from water. An experimental setup was devised to assess the removal efficiency of D-DTC-OP, along with its sensing capacity. A 100 ppm Cu²⁺ solution was prepared from copper nitrate. At first, 10 mg of D-DTC-OP was taken in 2 mL of water in a glass vial of water and was sonicated for 5 min to get a uniformly dispersed solution (Figure 4a). Following that, the solution was treated with 2 mL of a 100 ppm copper nitrate solution (overall Cu²⁺ concentration in the solution is 50 ppm); immediately, the solution exhibited a bright yellow color (Figure 4b), which shows that the polymer can detect the presence of Cu²⁺ in contaminated water. After a few minutes, it was observed that the colored particles had precipitated, leaving behind a clear transparent solution (Figure 4c). In the following step, the solution was filtered over a syringe filter, and this resulted in a transparent clear solution (Figure 4d). The complete removal of all Cu²⁺ ions from the solution by D-DTC-OP was verified by conducting a comparative study. The analysis was carried out in separate vials labeled “A” and “B”, onto which 150 μL from a 50 ppm Cu²⁺ solution was taken in vial “A”, while 150 μL from the supernatant was taken in vial “B” (Figure S29). A 10 μL aliquot of a D-DTC-OP solution in DMSO (15 mg of D-DTC-OP in 1 mL DMSO) was added to each of the above solutions, and changes were observed. It was observed that solution “A” turned yellowish on addition of the polymer, indicating that our polymer could detect the presence of Cu²⁺ ions at the ppm level. On the other hand, the color of solution “B” remained unchanged by the addition of D-DTC-OP, confirming that our polymer could efficiently remove all of the Cu²⁺ ions during the experiment, resulting in a Cu²⁺-free solution. After removal, the quantification of metal ions was done by the polymer sensor, which shows the removal of 99.81% of Cu²⁺ from the solution. Furthermore, to confirm the complete removal of the element from the solution, the ICP-MS (inductively coupled plasma mass spectrometry) technique was employed. The advantage of the technique is that it can accurately quantify the trace amount of metals. From the ICP-MS experiment, it was observed that the removal efficiency of the polymer for Cu²⁺ is 99.94% (Table S1).

Figure 4.

Removal of copper (Cu²⁺) from water.

A similar experiment was performed for the detection and removal of Co²⁺, Ni²⁺, Fe²⁺, and Fe³⁺. The procedure was the same as that in copper, with the difference in the source of stock preparation and the color imparted by addition of a polymer. The stock solutions of cobalt were prepared from cobalt nitrate, nickel from nickel chloride, iron (Fe²⁺) from ferrous sulfate, and iron (Fe³⁺) from ferric chloride, respectively. The stock solution became a green color for cobalt, a pale yellowish-green color for nickel, and a brown color for iron on addition of a polymer sensor (Figures S30–S33). After the treatment of the polymer with the metal solutions, the sensor was reintroduced. There was no change in the color for the cobalt and nickel ions, but there was a change in the color for the iron (Fe²⁺ and Fe³⁺) ions, which means the iron could not be removed from the water. In the same manner, the ICP-MS experiment was conducted, and it was determined that the removal percentage of 10 mg of the polymer from metal solutions containing 50 ppm concentrations of Ni²⁺ is 99.97% and Co²⁺ is 99.98% (Table S1). The uptake capacity was calculated by fitting with the Langmuir adsorption isotherm models (linear and nonlinear). The polymer exhibited a maximum adsorption capacity of 192.30 mg/g for Cu²⁺, 159.74 mg/g for Co²⁺, and 181.81 mg/g for Ni²⁺ (Figures S34–S36). The polymer maintains its effectiveness in removing metals at acidic, neutral, and basic pH levels (Figure S37). Reusability is essential for a material to make it suitable for practical use. The reusability of the polymer was achieved through a washing process, using hydrochloric acid (0.1 mol/L), followed by washing with water until the pH of the polymer solution became neutral. It was found that the polymer maintains around 80% of its efficacy in removing the metal, even after four cycles (Figure S38).

Conclusions

A novel fluorescent organic polymer (D-DTC-OP) was designed and synthesized by incorporating two key functionalities, namely, a dansyl fluorophore and dithiocarbamate (DTC). The deliberate integration of these two groups in the design of the organic polymer led to the discovery of customized physical and chemical properties that can be utilized in materials science. The amalgamation of the dansyl probe and dithiocarbamate leads to the formation of an organic polymer with superior optical and ligand properties. The finest advantage of D-DTC-OP is its dual functionality, allowing for both visual detection of heavy metals and the effective removal of these contaminants from water. From our studies, it was inferred that the dansyl moiety in the polymer contributes to its sensing application, whereas the DTC system is responsible for the removal of those toxic metals. The material was subjected to different characterization techniques that allowed structural elucidation and morphological understanding. The organic polymer was found to be crystalline and solid-state fluorescent; hence, its potency can be utilized in the field of optoelectronics, preferably in the fabrication of OLEDs. The material also shows an appreciable degree of thermal stability. The polymer removed Cu²⁺, Co²⁺, and Ni²⁺ with uptake capacities of 192.30, 159.74, and 181.81 mg/g, respectively, and could selectively sense these with visually distinct color changes specific to each metal, such as Cu²⁺ resulted in a yellow, Co²⁺ showed a green, Ni²⁺ displayed a pale yellowish-green, and Fe²⁺/Fe³⁺ exhibited a brown color. Hence, D-DTC-OP can be used as an excellent material to efficiently remediate our water bodies and restore the quality of the “blue gold”.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acspolymersau.4c00048.

• Details of synthesis and characterization of the monomer and the polymer by ¹H NMR, photophysical studies, TG-DTA, paper-strip sensing, and limit of detection calculation (PDF)

Author Contributions

M.P. and D.B. conceptualized the design of a fluorescent DTC-linked organic polymer. D.B. designed the synthetic strategy. D.B. and A.A. carried out all of the synthesis, characterization, and applications. D.B., A.A., and M.P. analyzed the data and wrote the paper. All authors have given approval to the final version of the manuscript. CRediT: Debashis Barik data curation, formal analysis, methodology, validation, visualization, writing-original draft, writing-review & editing; Abhirami Anilkumar data curation, formal analysis, validation, writing-original draft, writing-review & editing; Mintu Porel conceptualization, formal analysis, funding acquisition, investigation, project administration, resources, supervision, validation, writing-original draft, writing-review & editing.

This research was funded by the Indian Institute of Technology Palakkad, India; the Ramanujan Fellowship (SB/S2/RJN-145/2017), Science and Engineering Research Board, Department of Science and Technology, India; the Core Research Grant (CRG/2019/002495), Science and Engineering Research Board, Department of Science and Technology, India; and the Scheme for Transformational and Advanced Research in Sciences (MoE/STARS-1/293), Ministry of Education, India.

The authors declare no competing financial interest.

Special Issue

Published as part of ACS Polymers Auvirtual special issue “Polymer Science and Engineering in India”.