Hyperbranched polyethylenimine-based sensor of multiple metal ions (Cu²⁺, Co²⁺ and Fe²⁺): colorimetric sensing via coordination or AgNP formation

Abstract

Hyperbranched polyethylenimine (HPEI), an amine rich polymer, has been used as a colorimetric probe for selective sensing of multiple metal ions (Cu²⁺, Co²⁺ and Fe²⁺) with clearly distinguishable color in aqueous solution. The selective coordination of Cu²⁺ with the chelating amines of HPEI induced a visible color change from colorless to blue across a wide pH range (pH 4.0 to 10.0). Other metal ions did not show any significant color change. Interestingly, the addition of Co²⁺ and Fe²⁺ into HPEI in the presence of silver ions (Ag⁺) leads to the formation of strong yellow and brown colors, respectively. The control studies suggest that Co²⁺ and Fe²⁺ undergo oxidation and reduce the silver ions into silver nanoparticles (AgNPs). The strong surface plasmon resonance (SPR) vibration of the AgNPs was responsible for the yellow/brown color. UV-visible studies showed a strong absorption peak at 400 nm for Co²⁺ and a broad absorption with λ_max at 420 nm for Fe²⁺. HR-TEM studies confirmed more uniform spherical AgNPs for Co²⁺ with HPEI–Ag⁺ and polydisperse AgNP aggregates for Fe²⁺. The limit of detection of the HPEI probe for Cu²⁺ is 0.25 μg L⁻¹ and those of the HPEI–Ag⁺ probe for Co²⁺ and Fe²⁺ are 40 and 30 μg L⁻¹, respectively. The practical application of the HPEI probe for selective sensing of multiple metal ions with distinguishable colors has been demonstrated on Whatman filter paper. Thus, simple commercially available HPEI has been successfully used for the selective colorimetric sensing of biologically important Cu²⁺, Co²⁺ and Fe²⁺ metal ions in aqueous medium.

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Introduction

Cobalt (Co²⁺), iron (Fe²⁺) and copper (Cu²⁺) are important transition metal ions that play significant roles in human health, animals and plants. Co²⁺ is an essential micronutrient for animals and plants, and an important component of vitamin B₁₂ and other biological compounds.¹ However, deficiency of Co²⁺ can cause anaemia, slower growth and loss of appetite.² Also, higher contamination of Co²⁺ in environmental water can have severe effects on both humans and animals.³ At higher concentration, Co²⁺ is highly toxic to humans and can induce several diseases and disabilities that include asthma, decreased cardiac output, cardiac enlargement, heart disease, lung disease, dermatitis and vasodilation.⁴ Fe²⁺, the most abundant metal ion in plants and the human body, plays an important role in metabolism, catalysis, oxygen transport and as a cofactor in many enzymatic reactions.⁵ The imbalance of Fe²⁺ can induce several diseases, including anaemia, heart disease, liver damage and diabetes.^6–8 Similarly, copper (Cu²⁺) is the third essential transition metal ion for human health after zinc and iron and is an important cofactor for many enzymes in biological processes.^9–11 However, it is highly toxic for many organisms at higher concentrations because it may promote the generation of reactive oxygen species (ROS).¹² ROS strongly interfere with cellular signalling processes and cause damage to cell structures or provoke apoptosis.^13–16 Further, a high concentration of Cu²⁺ may also induce gastrointestinal disorders, as well as damage the liver and kidney.¹⁷ An increase in Cu²⁺ concentration in the neuronal cytoplasm might produce several other diseases like Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis.^18,19 The selective binding of Cu²⁺ with the histidine-rich regions of the prion protein lead to mis-folding and protein fibrilization.²⁰ According to the U.S. Environmental Protection Agency (EPA), the limit for the level of Cu²⁺ ions in drinking water is 1.3 mg L⁻¹, which is approximately 20 μM.²¹ The increased use of copper in electronic device manufacturing and industrial and agricultural processes makes Cu²⁺ contamination in environmental water a serious problem.²² Thus, developing methods for the continuous monitoring and detection of the concentration levels of Co²⁺, Fe²⁺, and Cu²⁺ in environmental water samples is imperative for the benefit of human health.

Traditional detection methods, such as inductively coupled plasma atomic emission spectrometry, atomic absorption spectroscopy and electrochemical methods, require sophisticated instrumentation, tedious sample preparation and high cost.^23–26 In contrast, colorimetric sensors that offer a naked-eye detectable visible color change upon interaction with metal ions have several advantages such as simple operation, cost-effectiveness, robustness and enabling of on-site monitoring.^27–32 Several Schiff base and naphthalimide-based chemosensors have been synthesized for selective colorimetric sensing of Cu²⁺ ions.^33–37 Surface functionalized noble silver nanoparticles (AgNPs) with specific organic linkers have also been reported as Cu²⁺ colorimetric sensors.^38–41 Molecular materials, predominantly Schiff bases, and rarely surface-functionalized noble nanoparticle-based optical sensors have been reported for Fe²⁺/Fe³⁺.^42–48 However, colorimetric sensors for Co²⁺ metal ions are rarely reported. A coumarin-conjugated thiocarbanohydrazone colorimetric sensor was developed by Debabrata Maity et al.⁴⁹ The Cheal Kim group reported a Schiff base colorimetric sensor for Co²⁺.⁵⁰ Triazole-carboxyl functionalized AgNPs and glutathione-functionalized Ag nanorods have also been reported for the selective sensing of Co²⁺ ions.^51,52 Recently, we have developed phenolic chelating ligand-functionalized AgNPs for the selective sensing of Co²⁺ ions in water.⁵³ Nevertheless, single probes that can detect more than two metal ions are scarcely reported.^43,54–56 Thus, developing a single colorimetric probe that can exhibit sensing of multiple metal ions in aqueous solution is highly challenging.

Herein, we have demonstrated highly selective colorimetric sensing of Cu²⁺, Fe²⁺ and Co²⁺ ions using commercially available hyperbranched polyethylenimine (HPEI) in aqueous medium. Addition of Cu²⁺ into the HPEI solution produced a visible color change (colorless to blue color) via selective coordination with chelating amine functionalities. The selective colorimetric change for Cu²⁺ ions can be observed clearly across a wide pH range (pH 4 to 10). Other metal ions did not show significant color change. Addition of Co²⁺ and Fe²⁺ ions into the HPEI polymer in the presence of Ag⁺ converted the colorless solution into yellow and dark brown colors, respectively. The mechanistic studies indicate that oxidation of Co²⁺ and Fe²⁺ reduces silver ions into AgNPs selectively and produces the yellow/brown color due to strong surface plasmon resonance (SPR). Absorption studies showed a strong absorption peak at 400 nm for Co²⁺ and a slightly broad absorption with λ_max at 420 nm for Fe²⁺. Importantly, both solution colours and absorptions are clearly distinguishable. HR-TEM studies confirmed the formation of more uniformly sized AgNPs for Co²⁺ and polydisperse AgNP aggregates for Fe²⁺. The concentration dependent studies showed a linear enhancement of absorption intensity with increasing Cu²⁺, Co²⁺ and Fe²⁺ concentration. To demonstrate the practical applicability of the HPEI probe, selective colorimetric sensing of Cu²⁺, Co²⁺ and Fe²⁺ has been performed on Whatman filter paper, which also exhibited clear distinguishable colors for the three metal ions. Thus, a simple HPEI colorimetric probe exhibited selective sensing of multiple metal ions with clearly distinguishable colors via coordination or oxidation-induced AgNP formation.

Experimental section

Poly(ethylenimine), branched ethylenediamine (hyperbranched polyethylenimine) (number average molecular weight, M_n = 600, weight average molecular weight, M_w = 800), poly(vinyl alcohol) (PVA), and poly(acrylic acid) sodium salt (PAA) were purchased from Sigma-Aldrich and used as received. Metal salts (Cr₂(SO₄)₃, Al(NO₃)₃, FeSO₄, CoCl₂, NiCl₂, Cu(OAc)₂, Zn(OAc)₂, Ca(NO₃)₂, Mg(OAc)₂, Cd(OAc)₂, HgCl₂, Pb(NO₃)₂, AgNO₃) were obtained from Ranbaxy India. The aqueous metal salt solutions (10⁻³ M) for colorimetric sensing studies were prepared using Milli-Q water.

HPEI colorimetric sensor studies

An aqueous solution of HPEI polymer was prepared by dissolving 0.25 g of HPEI in 100 mL of water. Each aqueous solution of metal ions (10⁻³ M) was added into 2 mL of HPEI polymer solution and the absorption and visible color change were monitored.

HPEI–Ag⁺ colorimetric sensor studies

To the above prepared HPEI polymer solution (20 mL), silver nitrate (10⁻³ M, 2 mL) was added and stirred at room temperature. This solution was kept in the dark as a stock solution. The colorimetric sensor studies for various metal ions were performed by adding each aqueous solution of metal salt (10⁻³ M) into the HPEI solution (1 mL) and monitoring the visible color change. The absorption spectra were recorded to support the color change.

Characterization

The UV-visible measurements of HPEI and HPEI–Ag⁺ with different metal ions were performed in a Perkin Elmer model Lambda 1050. The size and morphology of the AgNPs were investigated using high resolution transmission electron microscopy (HR-TEM). Samples for TEM measurements were prepared by placing a drop of NP solution on the graphite grid and drying it under vacuum. Transmission electron micrographs were taken using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV with an ultra high-resolution pole piece. The detection limit was calculated on the basis of absorption titration. To obtain the slope, the change of absorption intensity (640 nm for Cu²⁺, 400 nm for Co²⁺ and 418 nm for Fe²⁺) was plotted against the concentration of metal ions.

Result and discussion

Colorimetric sensor studies using HPEI

HPEI is an amine-rich polyamine–imine polymer with a metal-chelating structure (Scheme 1). The good coordination character of amines coupled with the chelating structure might promote coordination with transition metal ions. HPEI (0.25 g) was dissolved in water (100 mL). The pH of the as-prepared aqueous solution was 10.0. To investigate the colorimetric sensing, aqueous solutions of different metal ions (10⁻³ M) were added into the HPEI solution and the absorption and visible color change were monitored. Interestingly, Cu²⁺ addition into the HPEI solution selectively changed the colorless solution to a clear blue color (Fig. 1a). Addition of other metal ions into the HPEI solution did not result in any significant color change. Absorption studies of HPEI with Cu²⁺ showed a broad absorption peak around 650 nm (Fig. 1a). The appearance of a longer wavelength absorption was responsible for the formation of the blue color. It is noted that the HPEI solution is colorless and did not show any absorption in the visible region. Other metal ions with HPEI also did not exhibit any absorption in the visible region. The concentration dependence studies of Cu²⁺ with HPEI are shown in Fig. 1b. Addition of Cu²⁺ (10⁻⁴ M) clearly leads to the appearance of a longer wavelength absorption. The increasing concentration of Cu²⁺ resulted in a linear increase in absorption intensity. The interference studies of HPEI sensing of Cu²⁺ (μM) in the presence of different metal ions (10⁻³ M) confirmed the good selectivity (Fig. S1†). Cu²⁺ addition into HPEI in the presence of other metal ions also produced the longer wavelength absorption, which indicates other metal ions did not show significant interference in the Cu²⁺ colorimetric sensing.

Scheme 1 HPEI chemical structure and possible coordination with metal ions.

Fig. 1 (a) Digital image showing color change and absorption spectra of HPEI with different metal ions (10⁻³ M) and (b) HPEI absorption vs. concentration of Cu²⁺ (pH = 10).

The pH of the HPEI polymer solution was gradually decreased (8, 6, 4 and 2) by adding dilute HNO₃ to explore the colorimetric sensing of HPEI over a wide physiological range. Interestingly, addition of Cu²⁺ to the HPEI solution at pH 8.0 also clearly exhibited a blue color (Fig. S2†). Similarly, pH 6.0 and 4.0 solutions also showed clear formation of a blue color by adding Cu²⁺ ions. The absorption spectra of HPEI with Cu²⁺ at pH 8.0, 6.0 and 4.0 also showed longer wavelength absorption (Fig. 2 and S2†). However, the pH 2.0 polymer solution did not show any color change with Cu²⁺. It was observed that the pH 4.0 polymer showed a slightly reduced intensity of blue color for Cu²⁺ relative to pH 8.0 and 10.0. This could be due to the protonation of the free amine groups of HPEI at low pH, which might reduce their ability to coordinate strongly with Cu²⁺ ions. At pH 2.0, most of the amines might be protonated and hence could not form any coordination complexes with Cu²⁺ ions.

Fig. 2 Digital image showing color change and absorption spectra of HPEI with different metal ions at pH (a) 4.0 and (b) 6.0.

The addition of silver ions (Ag⁺) into the HPEI solution at pH 10.0 did not result in any color formation (Fig. 1a). However, formation of a light yellow to clear yellow color was observed in the presence of Ag⁺ upon reducing the pH of the HPEI solution from 10.0 to 6.0 (Fig. 2 and S2†). Further acidification (pH 4.0 and 2.0) produced only a very light yellow color. The appearance of the clear yellow color at pH 6.0 could be due to the formation of AgNPs. Absorption studies of HPEI with silver ions at different pH values showed a clear absorption peak at 400 nm for the pH 6.0 sample (Fig. 2b). A weak, slightly broad absorption was observed around 400 nm for the pH 4.0 and 8.0 samples. HPEI–Ag⁺ at pH 10.0 did not show any absorption in the visible region. The strong absorption at 400 nm suggests the formation of AgNPs. Noble metal nanoparticles such as silver (Ag) and gold (Au) are known to exhibit strong absorption in the visible region due to surface plasmon resonance (SPR) vibration.⁵⁷ AgNPs with a spherical shape generally exhibit an absorption peak between 390–440 nm. HR-TEM studies of Ag⁺ added to the HPEI sample at pH 6.0 clearly confirmed the formation of poly-disperse spherical AgNPs with a size range between 8 and 25 nm (Fig. 3a). HPEI–Ag⁺ at pH 10.0 did not show any AgNP formation which supports the absorption studies (Fig. 3b). It has been reported that polyethylenimine can reduce silver ions into AgNPs under acidic conditions whereas they remain as silver ions under basic conditions and formaldehyde reduction produced silver nanoclusters (AgNCs).^58,59

Fig. 3 HR-TEM images of HPEI–Ag⁺ at pH (a) 6.0 and (b) 10.0.

Colorimetric sensing of Co²⁺ and Fe²⁺ by HPEI–Ag⁺

The addition of Co²⁺ and Fe²⁺ into the HPEI polymer solution did not result in any significant color change across basic to acidic conditions. The absorption spectra also did not show any characteristic absorption peaks in the visible region (Fig. 1). However, the addition of Co²⁺ and Fe²⁺ into HPEI in the presence of Ag⁺ under ambient conditions (air equilibrated solution) produced clear yellow and dark brown colors from a colorless solution, respectively (pH = 10.0, Fig. 4). The light yellow color of the HPEI–Ag⁺ solution at pH 8.0 has also been converted to intense yellow and dark brown colors by Co²⁺ and Fe²⁺ addition (Fig. 4). Addition of other metal ions into HPEI–Ag⁺ did not result in any significant color change. The clear yellow color of HPEI–Ag⁺ at pH 6.0 was further intensified with Co²⁺ whereas Fe²⁺ addition again produced a dark brown color (Fig. 4). However, addition of Co²⁺/Fe²⁺ into HPEI–Ag⁺ at pH 2.0 and 4.0 did not result in any color change. Further, it was observed that the strong yellow color formed by Co²⁺ addition into HPEI–Ag⁺ at different pH values (6.0 to 10.0) was quite stable (for more than two weeks). The dark brown color of HPEI–Ag⁺ obtained by Fe²⁺ addition settled down slowly depending on the pH of the medium. At pH 10.0, the dark brown color settled down within 5 minutes, whereas it was stable for 30 minutes at pH 8.0. The dark brown color of the pH 6.0 sample was stable for 1 h and then slowly settled down. Thus, the AgNPs produced from HPEI–Ag⁺ with Co²⁺ and Fe²⁺ metal ions not only showed different colors but also showed different stabilities.

Fig. 4 Digital images of HPEI–Ag⁺ color changes upon addition of Co²⁺ and Fe²⁺ at different pH values.

The absorption studies of HPEI–Ag⁺ (pH = 8.0) with different metal ions also showed different absorption with Co²⁺ and Fe²⁺ (Fig. 5). Co²⁺ addition resulted in strong absorption at 400 nm, whereas Fe²⁺ addition to HPEI–Ag⁺ gave a broad absorption with λ_max at 420 nm. At pH 10.0, HPEI–Ag⁺ with Co²⁺ exhibited a clear absorption peak at 400 nm, whereas Fe²⁺ showed broad absorption without a clear peak (Fig. S3†). The broad absorption without a clear peak could be due to the faster settlement of the formed AgNPs. HPEI–Ag⁺ with Co²⁺ at pH 6.0 also exhibited a clear strong absorption at 400 nm (Fig. S4†). Co²⁺ addition strongly enhanced the absorption intensity without altering the λ_max. Fe²⁺ addition again resulted in broad absorption with λ_max at 418 nm. The strong yellow or brown color with clear absorption in the range between 400 to 440 nm indicates the formation of AgNPs. HR-TEM studies of HPEI–Ag⁺ (pH = 10), which is a colorless solution, did not show any AgNP formation (Fig. 3). Co²⁺ addition to HPEI–Ag⁺ (pH = 10.0) led to the formation of spherical AgNPs in the size range between 10 and 25 nm (Fig. 6). HR-TEM studies further indicate that Co²⁺ ions could coordinate with the HPEI amines and form spherical micron-sized particles (Fig. S5†). The presence of smaller AgNPs has also clearly been seen on the HPEI-Co²⁺ polymer microspheres. Fe²⁺ addition to HPEI–Ag⁺ (pH = 10.0), which produced a dark brownish color, showed the formation of polydisperse AgNP aggregates with different shapes (Fig. 6c and d and S6†). Spherical, nanorod and other morphologies of AgNPs with 10–40 nm size could be observed. Further, the HR-TEM suggests that the AgNPs are completely trapped in the polymer matrix. The formation of AgNPs with different sizes, shapes and aggregates was responsible for the different colors with Co²⁺ and Fe²⁺. HPEI–Ag⁺ at pH 6.0 showed the formation of poly-disperse spherical AgNPs (Fig. 3). However, addition of Co²⁺ induced the formation of nearly uniformly sized spherical AgNPs (5–10 nm) that strongly enhanced the absorption intensity (Fig. 6b). Hence, the HR-TEM studies indicate that HPEI in the presence of Ag⁺ facilitates the oxidation of Co²⁺ and Fe²⁺ and the reduction of silver ions into AgNPs.

Fig. 5 Absorption spectra of HPEI–Ag⁺ with different metal ions (10⁻³ M) at pH 8.0.

Fig. 6 HR-TEM images of HPEI–Ag⁺ after the addition of Co²⁺ at pH (a) 6.0 and (b) 10.0 and Fe²⁺ addition at pH 10.0 (c and d).

HPEI–Ag⁺ at pH 8.0 was chosen as a representative example to perform the concentration dependence studies. The concentration dependence studies of Co²⁺ and Fe²⁺ with HPEI–Ag⁺ are shown in Fig. S7.† HPEI–Ag⁺ showed a very weak, broad absorption around 400 nm. Addition of Co²⁺ (10⁻⁴ M) immediately leads to the formation of a strong absorption peak at 400 nm. Further addition resulted in a linear increase of absorption intensity without changing the λ_max with increasing concentration of Co²⁺. Addition of Fe²⁺ into HPEI did not result in an immediate increase of absorption intensity (Fig. S7b†). However, subsequent addition resulted in an enhancement of intensity with a red shift of the absorption peak to 418 nm. The limit of detection calculation indicates that the HPEI–Ag⁺ probe can detect Co²⁺ and Fe²⁺ in aqueous solution up to 40 and 30 μg L⁻¹, respectively. The interference studies of HPEI–Ag⁺ sensing of Co²⁺ and Fe²⁺ (μM) in the presence of different metal ions (10⁻³ M) demonstrate the high selectivity (Fig. 7, S8 and S9†). Addition of Co²⁺ to HPEI–Ag⁺ in the presence of other metal ions clearly produced a strong yellow color and absorption at 400 nm. Similarly, addition of Fe²⁺ to HPEI–Ag⁺ also produced a brownish color in the presence of other metal ions. It is noted that the color was lightened in the presence of other metal ions but a clear brown color was still produced. The interference studies indicate that HPEI–Ag⁺ has strong selectivity for Fe²⁺ in the presence other metal ions (Fig. S9†). Addition of Fe²⁺ (0.3 equivalent to Co²⁺) to HPEI–Ag⁺ with Co²⁺ increased the intensity with a small red shift; however, an equal amount of Co²⁺ (1 : 1 to Fe²⁺) was required to induce a small blue shift in the absorption. Interestingly, HPEI–Ag⁺ produced a strong reddish solution in the presence of both Fe²⁺ and Co²⁺ that is different from Fe²⁺ addition to HPEI–Ag⁺ or Co²⁺ addition to HPEI–Ag⁺. Further, the absorption spectra also showed a small red (400 to 412 nm for Fe²⁺ addition) or blue shift (420 to 410 nm) in the λ_max with peak broadening (red shift of λ_cut-off). Hence, although HPEI–Ag⁺ exhibits high selectivity for Fe²⁺, it can also be used as a colorimetric probe for detecting Co²⁺ metal ions.

Fig. 7 Selectivity studies of HPEI-Ag⁺ for (a) Co²⁺ (μM) and (b) Fe²⁺ (μM) in the presence of different metal ions (mM).

To get insight into the mechanism of Co²⁺ and Fe²⁺ colorimetric sensing by HPEI–Ag⁺, control studies were performed. It is noted that Co²⁺ and Fe²⁺ addition to the HPEI polymer alone did not result in any visible color or absorption in the visible region. However, Co²⁺/Fe²⁺ addition into HPEI in the presence of Ag⁺ under ambient conditions led to the appearance of a clear yellow/dark brown color due to the formation of AgNPs, which was confirmed by absorption and HR-TEM studies. The same experiment was performed under N₂ atmosphere. Interestingly, Co²⁺ and Fe²⁺ addition under inert conditions did not produce any color, which indicates that the Ag⁺ ions have not been reduced into AgNPs (Fig. S10†). However, removing the N₂ atmosphere and bringing the reaction medium into normal conditions resulted in the formation of yellow and dark brown colors immediately. Further, it was observed that Co³⁺ or Fe³⁺ addition to HPEI–Ag⁺ did not produce any color. Similarly, the direct addition of Co²⁺ or Fe²⁺ to Ag⁺ (10⁻⁴ or 10⁻³ M) without HPEI polymer under ambient conditions also did not result in characteristic color formation. Interestingly, addition of the HPEI polymer into the solution of Co²⁺/Fe²⁺ with Ag⁺ leads to the formation of a yellow/dark brownish color. Other water soluble polymers, such as poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP), were used to confirm the significance the HPEI polymer. Addition of Co²⁺/Fe²⁺ into aqueous solutions of PVA–Ag⁺ or PVP–Ag⁺ did not produce any significant color, which indicates the importance of the amine-rich HPEI polymer for the reduction of Ag⁺ to AgNPs (Fig. S11†). These studies indicate that Co²⁺/Fe²⁺ undergo oxidation in the presence of the HPEI polymer, Ag⁺ and oxygen in the air, which reduces the silver ions into AgNPs (Scheme 2). However, to induce Co²⁺/Fe²⁺ oxidation, Ag⁺, HPEI and oxygen are required. It appears that it might be a synergistic effect of all three components (Ag⁺, HPEI and air) resulting in the oxidation of Co²⁺/Fe²⁺ and reduction of Ag⁺ into AgNPs. It is noted that Co²⁺/Fe²⁺ oxidation will lead to the formation another stable oxidation state of these metal ions (Co³⁺/Fe³⁺). Importantly, both Co²⁺ and Fe²⁺ induced AgNPs showed distinguishable colors with different absorptions, which suggests that the HPEI–Ag⁺ probe can be used to detect Co²⁺ and Fe²⁺ ions in aqueous solution.

Scheme 2 Proposed mechanism of Co²⁺ and Fe²⁺ colorimetric sensing of HPEI–Ag⁺ via oxidation-induced AgNP formation.

The practical application of the HPEI probe for selective colorimetric sensing of Cu²⁺, Co²⁺ and Fe²⁺ in aqueous solution has been demonstrated using Whatman filter paper (Fig. 8). A droplet of aqueous solution of HPEI was placed on a small piece of filter paper. The addition of different concentrations of Cu²⁺ clearly produced a blue color in the filter paper. Similarly, addition of Co²⁺ and Fe²⁺ to HPEI–Ag⁺ soaked filter paper led to the formation of yellow and dark brown colors, respectively. The increasing Cu²⁺, Co²⁺ and Fe²⁺ concentrations led to higher intensities of color.

Fig. 8 Digital images of colorimetric sensing of Cu²⁺, Co²⁺ and Fe²⁺ by the HPEI probe using Whatman filter paper.

Conclusion

In conclusion, we have demonstrated a highly selective colorimetric sensing of multiple metal ions (Cu²⁺, Co²⁺ and Fe²⁺) using commercially available HPEI polymer in aqueous medium. HPEI showed selective appearance of a blue color upon addition of Cu²⁺ ions via coordination with amine functionalities of the polymer. Interestingly, the color change for Cu²⁺ could be observed across a wide pH range (pH 4.0 to 10.0). Other metal ions did not show any characteristic color change with HPEI. Addition of Co²⁺ and Fe²⁺ into the HPEI solution in the presence of Ag⁺ under ambient conditions produced strong yellow and dark brown colors, respectively. The control studies indicate that Co²⁺ and Fe²⁺ undergo oxidation under ambient conditions in the presence of Ag⁺ and HPEI polymer and reduce the silver ions into AgNPs. The strong SPR phenomena of the AgNPs produced the color. The formation of AgNPs was confirmed by absorption and HR-TEM studies. The different colors for Co²⁺ and Fe²⁺ were due to the change in size, shape and aggregation of the AgNPs. The limit of detection of the HPEI probe for Cu²⁺ is 0.25 μg L⁻¹ and those of the HPEI–Ag⁺ probe for Co²⁺ and Fe²⁺ are 40 and 30 μg L⁻¹, respectively. Interference studies confirm the high selectivity of the HPEI colorimetric probe for Cu²⁺, Co²⁺ and Fe²⁺. Importantly, all three metal ions produced completely different and distinguishable colors. The practical applicability of HPEI for the selective colorimetric sensing of Cu²⁺, Co²⁺ and Fe²⁺ has been demonstrated using Whatman filter paper.

Acknowledgements

Financial support from the Department of Science and Technology, New Delhi, India (DST Fast Track scheme no. SR/FT/CS-03/2011(G) and SR/FT/CS-09/2011) is acknowledged with gratitude. We thank CRF, SASTRA University for the UV-Visible spectrophotometer.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Absorption spectra, interference studies, digital images and HR-TEM images are included. See DOI: 10.1039/c5ra13797g

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