Rapid detection of mercury ions in water using functionalized liquid crystal microdroplets

Abstract

In this study, we present a stimuli-responsive liquid crystal (LC) droplet-based sensor system coated with poly(ethylene imine) (PEI) and Tween-20 for the detection of mercury ions (Hg²⁺) in aqueous environments. Upon exposure to Hg²⁺, the LC droplets undergo a distinct orientational transition from radial (homeotropic) to bipolar (planar), which can be clearly visualized using polarized optical microscopy (POM). This response is attributed to the strong interaction between mercury ions and the amine groups present in the PEI coating. The sensor exhibits a rapid response time of approximately 10 s at a mercury ion concentration as low as 59 mM. The limit of detection (LOD) in distilled water is determined to be 4.71 mM using a sample volume of 160 μL. The sensitivity of the system is influenced by parameters such as the concentration and volume of the mercury ion solution and the number of LC droplets used. Furthermore, the sensor demonstrates reliable detection of Hg²⁺ in various water samples, including distilled and lake water, indicating its practical applicability. In addition to detection, the PEI/Tween-20-coated LC droplets show potential for reducing mercury ion concentration through surface adsorption. This approach offers a simple, cost-effective alternative to conventional instrumentation for on-site mercury detection in environmental samples.

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1. Introduction

Water contamination by heavy metal ions has posed a severe global problem for decades. Heavy metals become highly toxic when they bind with protein thiol groups, leading to cellular penetration where they can cause oxidative damage, disrupt biological functions, and even lead to cancer. Among various metal ions, mercury (Hg²⁺) is recognized as one of the most toxic, particularly in environmental contexts, often resulting in significant water pollution. Freshwater bodies like lakes and rivers can accumulate high levels of Hg²⁺, which, through the food chain, can be readily absorbed by humans. Exposure to Hg²⁺ can lead to serious immune system impairment and organ damage. The accumulation of Hg²⁺ in the body is linked to conditions such as acrodynia, Minamata disease,¹ and Hunter–Russell syndrome,² along with irreversible harm to the kidneys,³ brain, and nervous system, which may ultimately result in fatality.^4,5 Traditional methods for quantifying Hg²⁺ ions include atomic fluorescence spectroscopy,^6,7 mass spectrometry,⁸ atomic plasma spectroscopy,⁹ and Raman spectroscopy.¹⁰ These procedures require sophisticated instrumentation and are often labour-intensive, hence generating a need for more speed, simple, and economical detection methods. DNA duplexes with T–T mismatches display a higher binding affinity for Hg²⁺ than A–T pairs, indicating their capacity to selectively sequester Hg²⁺ and establish stable T–Hg²⁺–T complexes, as documented by Ono and Togashi (2004).¹¹ Techniques employing this T-rich DNA mechanism, such as electrochemistry,¹² luminescence,^13,14 colorimetric assays,¹⁵ and Raman scattering,¹⁶ have demonstrated notable selectivity and stability in the detection of Hg²⁺.

Polyethyleneimine (PEI) is a water-soluble polymer known for its ability to selectively complex metal ions, a property recognized early on. Extensive research has explored its diverse applications, including use in the nuclear industry and in the removal of toxic heavy metal ions such as Pb(II), Cd(II), and Hg(II). As advancements in ligand design have yielded increasingly complex and selective complexing agents, attaching these reagents to polymer backbones has emerged as a promising approach to minimize the loss of costly compounds.¹⁷ Notably, PEI exhibits a higher absorption capacity for Hg²⁺ ions compared to Mn²⁺ and Cd²⁺ ions.¹⁸

Liquid crystals (LCs) have lately exhibited promise as innovative sensing systems.^19–21 The orientation of liquid crystal (LC) molecules is very susceptible to chemical and molecular interactions because of their extensive orientational order, resulting in enhanced optical signals.^22,23 LC-based sensors enable detection in ambient light without requiring electrical current or chemical tagging, generating optical signals visible to the naked eye. These characteristics make LC-based sensors ideal for applications requiring cost-effectiveness, high sensitivity, direct transduction, and simple detection.^24–30 A variety of biological and chemical compounds, such as DNA,^31–33 proteins,^34–40 enzymes,^41,42 proteases,^43,44 viruses,^45,46 and other analytes^47–60 have been detected using LC-based sensors. Research has examined liquid chromatography techniques for the identification of heavy metals. Hu et al. developed a liquid crystal (LC)-based sensor employing surface-immobilized urease to detect Cu²⁺ ions in aqueous solutions, leading to a transition in the LC's optical response from bright to dark due to urease inhibition by Cu²⁺ ions.⁶¹ Similarly, Jang et al. utilized 4-cyano-4-pentyl biphenyl (5CB) infused with stearic acid to detect Cu²⁺ and Co²⁺ ions by monitoring optical changes in liquid crystal alignment at the aqueous interface.⁶²

Recent advancements include LC-based methods for the detection of Hg²⁺ ions in aqueous solutions. Yang et al. created an LC biosensor utilizing oligonucleotide probes that modified LC orientation upon interaction with Hg²⁺, due to of oligonucleotide unique property to bind specifically to two DNA thymine (T) bases, is used as a model heavy metal ion.⁶³ Chen et al. developed an LC-based biosensor for the detection of mercury ions by using ligand-doped liquid crystal. The mercury ion is detected as a result of the interaction between the complex of ligand and Hg²⁺ formation.⁶⁴ Singh et al. developed a liquid chromatography-based sensor incorporating amphiphilic dithiocarbamate for the selective detection of Hg²⁺, It will be detected by LC as a result of the dithiocarbamate chelating group of MeDT with Hg²⁺ ion at the aqueous interface, which disrupted the orientation of LCs and produced a brilliant texture.⁶⁵

Hong et al. utilized aptamers in a liquid chromatographic sensor for the selective detection of Hg²⁺ ions. This sensor is based on interactions between trimethyl octadecyl ammonium bromide (OTAB) and Hg²⁺-binding aptamer.⁶⁶ Here we hypothesize that Hg²⁺ ions can be selectively and sensitively detected using nematic liquid crystal (NLC) droplets stabilized with polyethyleneimine (PEI) and Tween-20, without the need for oligonucleotides or other molecular recognition elements. Unlike previous LC-based sensors that require DNA probes or functionalized ligands for specificity, our simplified LC-based sensor design specifically detected Hg²⁺ ions in actual water sample (tap water) without doping oligonucleotide.

This study involved the generation of monodispersed nematic liquid crystal (NLC) droplets stabilized by a polyethyleneimine (PEI) and Tween-20 surfactant layer, enhancing droplet stability. Our research group previously utilized liquid crystal droplets stabilized with polyvinyl alcohol (PVA)/SC₁₂S for the detection of bile acids and bovine serum albumin (BSA)^39,67 and developed an economical method for the early detection of Alzheimer's disease using PEI/Tween-20 stabilized liquid crystal droplets.⁶⁸ We utilized PEI/Tween-20 coated LC droplets for the selective detection of Hg²⁺ ions in aqueous solutions. Due to the interaction between Hg²⁺ and the amine groups on PEI induces structural changes in the LC droplets, transitioning them from a homeotropic to planar configuration. These changes, observable under a polarized optical microscope (POM), allow Hg²⁺ ion detection by monitoring the transition from hedgehog (radial) to boojum (bipolar) configurations within fraction of the seconds. This label-free, simple, and cost-effective optical probe enables reliable detection of Hg²⁺ ions in aqueous solutions.

2. Experimental

2.1 Materials

Nematic liquid crystal E7 was purchased from Merck. Polyethyleneimine (PEI, 50 wt%), polyvinyl alcohol (PVA, molecular weight ∼160 000, HIMEDIA), sodium dodecyl sulfate (SC₁₂S, molecular weight ∼288.38, SRL), cetyltrimethylammonium bromide (CTAB, extrapure AR, 99%, molecular weight ∼364.45, SRL), coumarin 6 (98%, CAS no. 38215-36-0, Sigma-Aldrich), Tween-20, mercury(II) chloride (HgCl₂), magnesium chloride (MgCl₂), calcium chloride (CaCl₂), ferrous chloride (FeCl₂), zinc chloride (ZnCl₂), cadmium chloride (CdCl₂), copper chloride (CuCl₂), and sodium chloride (NaCl) were obtained from Sigma-Aldrich. PEI with amine groups was also purchased from Sigma-Aldrich. Additionally, Tween-20, lead nitrate, and all solvents were procured from SRL Chemicals. All reagents were of analytical grade and used without further purification.

2.2 Fabrication of flow-focusing microfluidic device

The flow-focusing microfluidic device was employed to produce monodispersed LC droplets. To construct this device, a tapered cylindrical capillary (inner diameter 0.58 mm, outer diameter 1.0 mm; borosilicate glass) was inserted into a square capillary (inner diameter 1.0 mm, wall thickness 0.2 mm; Square-Boro Tubing), as illustrated in Fig. 1. The left end of the cylindrical capillary was tapered using a Flaming micropipette puller (P-97, Sutter Instrument), and then precisely sanded to a 40 μm orifice with a Microforge (MF-830, Narishige). All capillaries were blown with air and rinsed with water to remove dust particles. Octadecyl trimethoxy silane (OTS) was applied to render the square capillary hydrophobic. The treated square capillary was then fitted with the tapered cylindrical capillary, and two needles were epoxy-glued to each end of the square capillary to facilitate fluid pumping without leakage.

Fig. 1 (a) Photograph of the flow-focusing microfluidic device. (b) Schematic diagram illustrating the direction of fluid flow within the microfluidic device.

2.3 Preparation of metal ion solutions

To investigate the response times as a function of mercury ion concentration, a series of mercury ion solutions were prepared in both distilled waters, tap water and lake water. Concentrations were varied systematically, including 4.71 mM, 8 mM, 9.8 mM, 10.31 mM, 14.8 mM, 22.1 mM, 29.6 mM, 36.9 mM, 44.2 mM, 51.6 mM, and 59 mM. This range of concentrations allowed for a thorough assessment of the sensor's responsiveness across varying mercury levels, offering valuable insights into how concentration influences the sensor's detection efficiency and response behavior in distilled water, tap water and lake water.

2.4 Characterization

A polarizing optical microscope (POM) (Olympus BX53, USA) in transmission mode, equipped with a CCD camera (Micropublisher RTV 5.0, Q-Imaging), was used to analyze the director arrangement of the liquid crystal (LC) within the droplets and capture textural images of the LC. Additionally, a fluorescence microscope (Olympus IX 73 inverted microscope, Japan) was employed to observe fluorescence images on the surface of the LC droplets. Chemical composition was assessed through Raman spectroscopy, with spectra recorded using an in Via Raman 785 ER system featuring a 785 nm laser and a charge-coupled device (CCD) detector, provided by Wasatch Photonics.

2.5 Detection of mercury ion (Hg²⁺) in aqueous environments

To detect mercury ions, a 4 μL droplet of the LC_PEI/Tween-20 emulsion was carefully deposited onto a clean glass slide. Different concentrations of mercury ions (Hg²⁺) were then introduced to observe the response time of the LC_PEI/Tween-20 droplets as they transitioned from a homeotropic to a planar configuration. This procedure was repeated for each concentration to ensure consistency and accuracy in the measurements.

3. Results and discussion

3.1 Selection of surfactants for mercury ion (Hg²⁺) detection

We investigated different surfactant combinations (PVA/SC₁₂S, PVA/CTAB, and PEI/Tween-20) to produce LC droplets using a vortex mixer, assessing their responsiveness to mercury ions (Hg²⁺) to identify the optimal surfactant for Hg²⁺ detection. Initially, LC droplets coated with PVA (1 wt%)/SC₁₂S (5 mM) were prepared, displaying a radial structure under a polarized optical microscope (POM). Upon immersing these LC droplets in an aqueous Hg²⁺ solution, their orientation shifted from radial to bipolar, visible under POM after approximately 10 min, as shown in Fig. 2. In the absence of Hg²⁺ ions, no structural transition was observed in the LC droplets. The Hg²⁺ solution was prepared in a buffer at pH 4.

Fig. 2 POM images of LC droplets coated with PVA (1 wt%)/SC₁₂S (5 mM): (a) before and (b) after the addition of Hg²⁺ ions (59 mM) in a buffer solution at pH 4.

We observed that LC droplets in tap water transitioned from homeotropic (radial) to planar (bipolar) alignment, irrespective of the presence of mercury ions. The observed behaviour is probably attributable to the pH sensitivity of the PVA/SC₁₂S surfactant coating,³⁹ resulting in orientation alterations in the droplets. Consequently, these droplets can only identify mercury ions in a buffer solution, but not in tap water. We additionally examined LC droplets coated with PVA (1 wt%)/CTAB (5 mM) and observed that they exhibited a comparable response to mercury ions in a buffer solution, akin to PVA/SC₁₂S-coated droplets. This indicates that PVA/CTAB-coated droplets proficiently identify mercury ions in the buffer, shifting from homeotropic to planar alignment within 10 min, as depicted in Fig. 3.

Fig. 3 POM pictures of LC droplets coated with PVA (1 wt%)/CTAB (5 mM): (a) prior to and (b) subsequent to the introduction of Hg²⁺ ions (59 mM) in a buffer solution at pH 4.

The LC droplets preserved a radial configuration even without the presence of mercury ions.

In tap water, the LC droplets transitioned from a homeotropic to a planar orientation, irrespective of the presence of mercury ions. To enable mercury ion detection in tap water, we tested LC droplets coated with PEI and Tween-20 in distilled water and buffer solutions. A rapid orientation changes from homeotropic (radial) to planar (bipolar) was observed within a few seconds (10 s), as shown in Fig. 4. Of the three surfactants tested, PEI/Tween-20-coated LC droplets proved most effective for detecting mercury ions in buffer solutions, distilled water, tap water (real water samples) and lake water. Therefore, we selected PEI (1 wt%)/Tween-20 (0.05 wt%) as the surfactant for mercury ion detection in aqueous solutions.

Fig. 4 POM images of LC droplets coated with PEI/Tween-20: (a) before and (b) after the addition of Hg²⁺ ions (59 mM) in a buffer solution at pH 4.

3.2 Production of monodispersed LC droplets and analysis

We conducted a detailed study with a nematic liquid crystal (E7) coated in PEI surfactant at a concentration of 1 wt%, which showed a bipolar structure.⁶⁹ When we combined different concentrations of Tween-20 (0.2, 0.1, and 0.05 wt%) with PEI (1 wt%) and applied this mixture to coat LC droplets, we observed a radial configuration (homeotropics) under polarized optical microscopy (POM).⁶⁸ As an amphiphilic molecule, Tween-20 integrates into the PEI layer coating the LC droplets, altering the orientation of the LC molecules perpendicular to the director and enhancing birefringence under POM, with a central defect visible without cross polarizers.

From these combinations, we selected PEI/Tween-20 (1 wt% PEI and 0.05 wt% Tween-20) as the optimal continuous phase for producing LC droplets. To achieve monodispersed LC droplets, we employed a flow-focusing microfluidic device, setting the continuous phase (PEI/Tween-20 solution) flow rate to 20 μL min⁻¹ and the dispersed phase (NLC) flow rate to 0.3 μL min⁻¹ at room temperature. This setup yielded monodispersed LC droplets with an average diameter of 85 μm. The PEI coating on the droplet surface prevents collapse, while Tween-20 facilitates the transition from planar (radial) to homeotropic (bipolar) anchoring. Consequently, the LC droplets display a homeotropic configuration with an average diameter of 85 μm (measure under POM), as shown in Fig. 5.

Fig. 5 POM images of PEI/Tween-20-coated LC droplets produced by a flow-focusing microfluidic device, showing an average droplet diameter of 85 μm in a homeotropic configuration: (a) with cross polarizers and (b) without cross polarizers.

3.3 Detection of mercury ion using LC_PEI/Tween-20 droplets

After confirming the homeotropic orientation of LC droplets coated with PEI/Tween-20 (1 wt% PEI, 0.05 wt% Tween-20) under polarized optical microscopy (POM), we proceeded to introduce an aqueous mercury ion solution to the PEI/Tween-20-coated LC droplets to observe their response. The time-lapse sequence of images captured under POM, as shown in Fig. 6 and 7(i)–(viii), alongside the attached video file, illustrates the dynamic changes in droplet configuration upon exposure to mercury ions.

Fig. 6 POM images of LC droplets coated with PEI/Tween-20 after the addition of Hg²⁺ ion aqueous solution at a concentration of 59 mM: (a) with cross polarizers and (b) without cross polarizers.

Fig. 7 POM images of LC droplets (85 μm diameter) coated with PEI/Tween-20, captured in transmission mode at intervals following the introduction of 40 μL of Hg²⁺ ion aqueous solution: (i) 0 s, (ii) 1 s, (iii) 3 s, (iv) 5 s, (v) 6 s, (vi) 8 s, (vii) 9 s, and (viii) 10 s.

Initially, the LC droplets displayed a well-defined homeotropic orientation with a fan-like structure, characteristic of their stable configuration in the absence of mercury ions. However, following the addition of mercury ions, the homeotropic orientation began to gradually transform. The fan-like structure diminished as the droplets adopted a bipolar configuration, signifying a planar alignment of the LC molecules within the droplets.

This shift from homeotropic to bipolar orientation is driven by the adsorption of mercury ions onto the surface of the LC_PEI/Tween-20 droplets. The mercury ions interact with the amine groups in the PEI layer, altering the surface anchoring conditions of the LC molecules. Consequently, this interaction disrupts the homeotropic alignment, promoting a planar configuration in response to mercury ion adsorption. This transition is clearly illustrated in Fig. 7, where the visual changes in droplet orientation provide a straightforward and label-free optical indicator of mercury ion presence, demonstrating the effectiveness of PEI/Tween-20-coated LC droplets for detecting mercury ions in an aqueous environment.

To confirm the specificity of LC_PEI/Tween-20 droplets for detecting Hg²⁺ ions in aqueous solutions, we investigated the orientational transition of these droplets upon the addition of various other metal ions (Mg²⁺, Fe²⁺, Cu²⁺, Ca²⁺, Cd²⁺, Pb²⁺, Zn²⁺, and Na⁺) to the sensor system. Even after 30 min at a concentration of 59 mM, none of these metal ions induced a structural transition in the LC_PEI/Tween-20 droplets, which retained their homeotropic configuration, as shown in Fig. 8(a)–(h). This indicates the selectivity and reliability of the sensor, as the LC_PEI/Tween-20 droplets consistently displayed a homeotropic structure for all tested ions except Hg²⁺. In the presence of Hg²⁺ ions, however, a clear transition from homeotropic to planar structure was observed in the aqueous solution, as illustrated in Fig. 8(i).

Fig. 8 POM images of LC_PEI/Tween-20 droplets after the addition of 59 mM aqueous solutions of various metal ions: (a) Mg²⁺, (b) Fe²⁺, (c) Cu²⁺, (d) Ca²⁺, (e) Cd²⁺, (f) Zn²⁺, (g) Pb²⁺, (h) Na⁺, and (i) Hg²⁺.

To assess the sensor's performance in a complex ionic environment for real-time applications, we carefully studied a mixture of other metal ions (e.g., Cu²⁺, Fe²⁺) in aqueous solution (tap water). In these mixed-ion environments, the LC_PEI/Tween-20 droplets showed no orientational transition, as illustrated in Fig. 9(a). However, upon introducing Hg²⁺ ions at a concentration of 59 mM to the same solution, the LC_PEI/Tween-20 droplets displayed a clear transition from homeotropic to planar orientation within 15 s. This result demonstrates that Hg²⁺ ions in aqueous solution can be effectively detected through the homeotropic-to-planar transition in LC droplets as shown in Fig. 9(b).

Fig. 9 (a) POM images of LC_PEI/Tween-20 droplets after addition of (a) Cu⁺², Fe⁺² (b) Cu⁺², Fe⁺², Hg⁺² ion (c) Schematic representation LC_{-PEI/Tween-20} droplets of transition after addition of mercury ion.

Driven by the particular sensitivity of LC_PEI/Tween-20 droplets to mercury ion adsorption, we sought to identify the fundamental detection mechanism. A PEI/Tween-20 surfactant layer covers the LC droplets; the surfactant consists in amine (NH₂) groups. These amine groups generate strong coordination complexes that enhance their attraction for metal ion by means of nitrogen atoms able to donate electrons.^70,71 This binding process reduces the tendency to form dative π-bonds between metal ion and nitrogen atoms, enabling two nitrogen atoms to coordinate with a single metal ion in solution. Here the formation of the Hg–amine complex disrupts the orientation of the liquid crystal droplets.

In this system, PEI acts both as a surfactant and as a chelating ligand for Hg²⁺ ions. Even minimal perturbation by mercury ions is sufficient to induce a transition in PEI/Tween-20-coated LC droplets from homeotropic to planar orientation. When Hg²⁺ ions are present in the aqueous solution, the LC_PEI/Tween-20 droplets transition from a homeotropic (radial) to a planar (bipolar) configuration, while with other metal ions, the droplets retain their radial orientation. This indicates strong specificity of the system for Hg²⁺ detection, attributed to the high binding affinity of the amine groups for mercury ions and also more adsorption power of PEI toward mercury ion (0.87 mmol g⁻¹) compare to the other metal ions (Mg²⁺, Fe²⁺, Cu²⁺, Ca²⁺, Cd²⁺, Pb²⁺, Zn²⁺, and Na⁺),^17,18 a finding consistent with other metal ions as shown in Fig. 8.

The proposed detection mechanism is illustrated schematically in Fig. 9(c). In the absence of Hg²⁺ or other metal ions (such as Mg²⁺, Fe²⁺, Cu²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Pb²⁺, and Na⁺), the LC_PEI/Tween-20 droplets display a homeotropic orientation due to the presence of Tween-20 amphiphilic molecules on their surfaces. When Hg²⁺ ions are introduced, however, the orientation shifts from homeotropic (radial) to planar (bipolar). This shift results from the interaction between PEI and mercury ions, which displaces the amphiphilic molecules from the LC droplet surfaces. A video demonstrating this detection mechanism is available in ESI,† Video S1.

3.4 Detection of mercury ion by dye-doped interface method

This experiment provides clear evidence of the interactions between a coumarin dye and mercury ions on liquid crystal (LC) droplet surfaces. We sought to examine the nature of interactions by applying a coumarin dye in conjunction with an aqueous mercury ion solution to the surfaces of liquid crystal droplets. Fluorescence microscopy revealed a significant finding: the coumarin dye efficiently adhered to the surfaces of the LC droplets, resulting in strong fluorescence emission. This effect may arise from the interaction between mercury ions and the LC_PEI/Tween-20-coated surfaces of the droplets, which appears to alter their orientation from homeotropic to planar. This change in orientation is illustrated in Fig. 10(a) and (b), which provides both bright-field and fluorescence pictures for comparison examination.

Fig. 10 POM images of LC_PEI/Tween-20 coated droplets with coumarin dye doped with mercury ions: (a) bright field, (b) fluorescence image.

3.5 Raman spectroscopy studies

Fig. 11 shows the Raman spectrum, offering a comprehensive examination of the molecular interactions occurring when mercury ions attach to the surfaces of PEI/Tween 20-coated liquid crystal droplets. The peak at 2230 cm⁻¹ is attributed to the stretching vibration of the carbonitrile (C≡N) bond.⁷² This peak signifies the presence of carbonitrile groups, which are integral to the chemical composition of the droplets. The peak at 1607 cm⁻¹ is indicative of the amide group, a component of the poly(ethylene amine) (PEI) coating on the LC droplets. This peak is significant as it exemplifies the impact of the coatings on the molecular structure, demonstrating how the amide group, through its unique vibrations, interacts with mercury ions. The amide functionality on the surface is essential for ion binding and anchoring, potentially stabilizing the droplet surface. The peak at 1289 cm⁻¹ indicates the presence of biphenyl rings in the liquid crystal.⁷² The biphenyl rings form the primary structure of the liquid crystal, providing stability and unique optical properties. The presence of this peak underscores the integrity of the liquid crystal structure, although its interaction with mercury ions, and offers insights on the alignment and orientation of the liquid crystal molecules. The peak at 1184 cm⁻¹ signifies C–H stretching vibrations, typical of organic compounds.⁷³ The C–H stretching represents a fundamental vibrational mode, offering insight into the structural composition of the coating and the liquid crystal core. A subtle yet identifiable peak at 808 cm⁻¹ indicates the ring breathing mode, a refined vibration within the aromatic rings. This peak signifies the interaction between the ring structures of the liquid crystal material and the mercury ions. The signal at 436 cm⁻¹ is attributed to C–C stretching in the aliphatic chain. This peak highlights the importance of the aliphatic chain in maintaining the structural integrity of the coated LC droplets. The signal at 327 cm⁻¹ unequivocally signifies the interaction of Hg²⁺ ions. This peak clearly indicates the vibrational response caused by the binding of mercury ions to the surfaces of the droplets. The observation confirms the successful binding of mercury ions, illustrating the specific chemical interaction between Hg²⁺ ions and the PEI/Tween 20-coated droplets. This peak is crucial, since it promptly illustrates the impact of mercury on the molecular architecture of the LC droplets, signifying a modification in their overall composition and behavior.

Fig. 11 (a) The Raman absorption spectra of LC_PEI/Tween-20 (red) and LC_PEI/Tween-20 with 59 mM (blue) and 29.5 mM (green) concentrations of mercury ion (b) zoom portion of Raman absorption spectra (250–560 cm⁻¹) represent Hg²⁺ ion and amine interaction.

3.6 Limit of detection (LOD)

The minimum concentration of Hg²⁺ ions required to induce a change in the orientation of PEI/Tween-20-stabilized LC droplets from homeotropic to planar is known as the limit of detection (LOD). To test the detection capability in real water samples, we prepared Hg²⁺ ion solutions in distilled water, tap water and lake water. For response time measurements, LC droplets with a diameter of approximately 85 μm were used. In distilled water, the response time decreases from 161.33 s to 10 s as the Hg²⁺ concentration increases from 14.8 mM to 59 mM as shown in Fig. 12(a). Similarly, in tap water, the response time decreases from 224.33 s to 19 s with the same increase in concentration. The response time of lake water decreases from 42 s to 12.66 s as the concentration increases from 36.9 to 59 mM. This indicates that as the concentration of mercury ions decreases, the response time increases, as observed in the system's LOD for detecting mercury ions by repeating experiments with varying Hg²⁺ concentrations. In the optical images, a noticeable change in droplet configuration from homeotropic to planar is observed. Fig. 12(b) shows that for an aqueous mercury ion solution, as the volume increases from 40 μL to 160 μL, the LOD decreases from 14.73 mM to 4.71 mM. Fig. 12(b) shows that for an aqueous mercury ion solution, as the volume increases from 40 μL to 160 μL, the LOD decreases from 14.73 mM to 4.71 mM, while response time increases from 161.33 s to 516 s. This suggests that as the volume of the Hg²⁺ ion solution increases, the LOD decreases, likely because a higher volume equates to more Hg²⁺ ions, thereby facilitating the transition of LC_PEI/Tween-20 droplets from homeotropic to planar orientation. The configuration transition from radial to bipolar was observed in PEI/Tween-20 droplets down to a mercury ion concentration of 4.71 mM. At concentrations below 4.71 mM (at volume 160 μL), no transition occurs even after 30 min of observation, indicating that this lower concentration does not contain enough Hg²⁺ ions to trigger the structural change.

Fig. 12 (a) Response time of LC_PEI/Tween-20 droplets with an average diameter of 85 μm at different concentrations of Hg²⁺ ion. (b) Response time with respect to varying volumes of aqueous mercury ion solution at different concentration of mercury ion introduced into LC_PEI/Tween-20 droplets with standard deviation and error bar.

Table 1 provides a comparison between the LC droplet sensing platforms for mercury ion detection. In this study, we used PEI/Tween-20 as a surfactant to produce LC droplets specifically for mercury ion detection in aqueous solutions. Other approaches, such as using aptamers, oligonucleotides, and amphiphilic dithiocarbamate (MeDTC), serve as recognition elements for mercury ion detection on LC-solid surfaces. In our work, we used PEI (1 wt%) and Tween-20 (0.05 wt%) as surfactants to create monodispersed LC droplets for mercury ion detection at the LC-aqueous interface, without the need for aptamer doping in the LC. Differences in detection limits and response times for mercury ions can be attributed to the diverse interactions between mercury ions and the detection probes, resulting in variable detection thresholds and response times.

Table 1 Comparison between the LC droplet sensing platforms for mercury ion detection

LC materials	Sensing plat form	Detection probe	Principle	LOD	Response time	Ref.
4-Cyano-4′-pentylbiphenyl (5CB)	LC-solid	Oligonucleotide	Formation T–Hg^2+–T complex	0.1 nM	1 h	63
4-Cyano-4′-pentylbiphenyl (5CB)	LC-solid	5-(Pyridine-4yl)-2-(5-(pyridin-4-yl) thiophen-2-yl) thiazole (ZT)	Formation of ZT–Hg^2+ complex	10 μM	10 min	64
4-Cyano-4′-pentylphenyl (5CB)	LC-solid	Amphiphilic dithiocarbamate (MeDTC)	MeDTC–Hg complex	0.5 μM	>5 min (tap water)	65
4-Cyano-4′-pentylphenyl (5CB)	LC-solid	Aptamer	Formation T–Hg^2+–T complex	100 pM	10 min	66
Nematic liquid crystal (E7)	LC-aqueous interface	Surfactant PEI/Tween-20	PEI–Hg complex	4.71 mM	10 s	Current work

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Additionally, Table 1 provides a comparative analysis of liquid crystal (LC)-based biosensors for the detection of mercury ions (Hg²⁺). In this study, LC droplets were employed without the incorporation of high-cost oligonucleotides to achieve selective detection of Hg²⁺ in drinking water. The response time was within 10 seconds, which is notably faster compared to other reported LC-based biosensors for Hg²⁺ detection. The PEI-coated LC droplets effectively reduced the concentration of Hg²⁺ due to the higher absorption capacity of PEI compared to other ions such as Mg²⁺, Fe²⁺, Cu²⁺, Ca²⁺, Cd²⁺, Zn²⁺, Pb²⁺, and Na⁺.¹⁸ Moreover, the PEI/Tween-20 coated LC droplets facilitated the reduction of metal ion concentrations in the aqueous medium by adsorbing the ions onto their surfaces. In an aqueous solution, PEI/Tween-20 coated LC droplets show a dual nature for metal ions.

4. Conclusion

In conclusion, we have successfully developed a liquid crystal (LC)-based optical sensor for the selective detection of mercury (Hg²⁺) ions in aqueous solutions. This system was realized by generating microdroplets of nematic LC coated with polyethyleneimine (PEI, 1 wt%) and Tween-20 (0.05 wt%), which initially exhibited a homeotropic orientation under polarized optical microscopy (POM). Upon exposure to Hg²⁺ ions, a distinct orientational change from homeotropic to planar alignment was observed at the aqueous interface. This transition is attributed to the strong interaction between the amine (–NH₂) groups of PEI and Hg²⁺ ions, resulting in the formation of a stable PEI–Hg²⁺ complex. These molecular-level interactions facilitated the selective recognition of Hg²⁺ and disrupted the LC alignment accordingly. Raman spectroscopy and fluorescence microscopy further validated these findings, confirming both the chemical interaction and the associated reconfiguration of the LC droplets. Using transmission-mode POM, we characterized the sensor's performance, achieving a rapid response time of 10 s at a Hg²⁺ concentration of 59 mM, and a detection limit of 4.71 mM in distilled water. This label-free, straightforward, and cost-effective sensing platform enables efficient detection of mercury ions with high specificity. Moreover, the versatility of this approach suggests its potential for adaptation to detect other hazardous heavy metal ions, broadening its utility for environmental monitoring and public health applications.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data will be made available on request to the corresponding author.

Acknowledgements

The authors would like to thank the Department of Physics and Central Instrument Facility (CRIF) at NIT Warangal for providing a Research facility. The authors acknowledge financial support from the Department of Biotechnology (DBT), New Delhi, India from Research grant no: BT/PR35841/MED/32/745/2020.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ma00260e

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