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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)

Membrane Journal Vol.31 No.4 pp.241-252
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2021.31.4.241

Nanofiber Membrane based Colorimetric Sensor for Mercury (II) Detection: A Review

Saeyun Bhang^*, Rajkumar Patel^**†

^*Bio-Convergence (BC), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 03722, South Korea
^**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, 85 Songdogwahak-ro, Yeonsu-gu, Incheon 03722, South Korea

^†Corresponding author(e-mail: rajkumar@yonsei.ac.kr; http://orcid.org/0000-0002-3820-141X)

Received August 19, 2021 ; Review August 26, 2021 ; Accepted August 26, 2021

Abstract

Rapid industrialization with growing population leads to environmental water pollution. Demand in generation of clean water from waste water is ever increasing by scarcity of rain water due to change in weather pattern. Colorimetric detection of heavy metal present in clean water is very simple and effective technique. In this review membrane based colorimetric detection of mercury (II) ions are discussed in details. Membrane such as cellulose, polycaprolactone, chitosan, polysulfone etc., are used as support for metal ion detection. Nanofiber based materials have wide range of applications in energy, environment and biomedical research. Membranes made up of nanofiber consist up plenty of functional groups available in the polymer along with large surface area and high porosity. As a result, it is easy for surface modification and grafting of ligand on the fiber surface enhanced nanoparticles attachment.

Key Words : colorimetric sensor , nanofiber membrane , mercury (II) , fluorescent probe

나노 섬유 멤브레인을 기반으로 한 수은(II) 색변화 검출 센서에 대한 총설

방 세 연^*, 라즈쿠마 파텔^**†

^*연세대학교 언더우드국제대학 융합과학공학부 바이오융합과
^**연세대학교 언더우드국제대학 융합과학공학부 에너지환경과학공학

초록

급격한 산업화와 인구수 증가로 인한 환경 수질 오염이 발생하고 있다. 더불어 날씨 패턴의 변화로 인해 빗물이 부족해지자, 폐수를 깨끗한 물로 재활용하기 위한 요구가 나날이 늘어나고 있다. 색변화를 이용한 수중 속 중금속 검출은 아 주 간단하고 효과적인 기술이다. 본 논문에는 멤브레인을 이용한 수은 이온 색검출에 대해 자세하게 논의되어 있다. 셀룰로 스, 폴리카프로락톤, 키토산, 폴리설폰 등의 멤브레인이 금속 이온 검출을 지지체로서 사용되었다. 지지체로서 사용된 멤브레 인들은 나노 섬유를 기반으로 하며 표면적이 크며, 중금속 검출의 활성 부위로 사용하기에 탁월하다. 나노 섬유를 기반으로 한 재료는 에너지, 환경, 그리고 바이오메디컬 연구에서 다양하게 응용될 수 있다. 나노 섬유로 이루어진 멤브레인들은 폴리 머에 있는 적용기를 많이 받아들일 수 있으며, 표면적이 넓고 다공성이라는 장점이 있다. 이로 인해 멤브레인의 표면 구조를 변화시키거나 리간드를 섬유 표면에 부착해 나노 입자 결합을 더 쉽게 해준다.

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1. Introduction

Among various heavy elements mercury (II) ion is one of the most hazardous metal ion [1-5]. Pollution of the environment by this element is mainly due to the effluents form chemical industry, mining industry, combustion engine running on fossil fuel, incineration of solid waste etc. Mercury ion is soluble in water and very stable. Accumulation in human organs leads to devastating effect on central nervous system as well as failure of organ function such as kidney failure, brain damage. Maximum amount of mercury ion that is allowed in the drinking water is 2.0 parts per billion. Detection of mercury level in the effluents are very important for human health.

Various metal ion detection methods are inductively coupled plasma-mass spectroscopy, atomic absorption spectroscopy and mass spectroscopy. But there is limitation due to cost factor, time taken for measurement as well as labor intensive. Visual detection of metal ions is one of most convenient method [6-11]. This method is based on interaction of gold nanoparticle with the metal ion know as surface enhanced plasmonic resonance method. Gold nanoparticle are not stable for long time. So there is limitation for sensing of metal ion in real time by this method.

Solid state sensor based on gold nanoparticles enhance the stability as well as durability [12-17]. Different types of support used for solid state sensors are porous wood, glass, paper made up of cellulose. Nanofiber membranes made up of cellulose, chitosan, polycaprolactone and polysulfone are discussed in this review. Fig.1 Schematic presentation of the classification of the review.

2. Mercury sensors

Gold nanomaterials materials have limitation in application for chemical detection due to lower fluorescence efficiency as compared to quantum dots or organic dyes. Modification of nanoparticles with silicon oxide (SiO₂) enhance the luminescent properties as well as generate higher quantum yield [18]. Metal organic framework (MOF) with nitrogen atoms have very good interaction with mercury ions resulting in better sensitivity [19].

2.1. Cellulose

In an attempt to develop a sensor that overcomes the biggest limitation of conventional solution-based Au nanomaterial sensors, which is stability, Fu et al. proposed the synthesis of solid-state membrane-based sensor for detection of Hg²⁺ [20]. Using porous cellulose nanofibrilatted (CNF) membrane as the platform for immobilization of gold nanoparticles (AuNPs) and sensing of Hg²⁺ ions, the AuNPs@CNF membrane showed a high fluorescent ability, where the lowest detection limit was determined to be 0.1 nM. The AuNPs@CNF sensor outshines other solution-based Au nanomaterial sensor when comparing their stabilities and AuNPs@CNF also exhibits great selectivity and facile detection method. Furthermore, substituting AuNPs for a different optical probe can potentially enable a detection of other metal ions, which suggests that CNF membrane can be used for a wide range of applications in real-site detections.

Porous cellulose acetate fiber is used as the support membrane for dithiothreitol capped with gold nanocluster (DDT.AuNC) which acts as fluorescent probe [21]. This is mainly used to detect Cu²⁺ ion, with a high selectivity for the specific metal ions. Selectivity of the sensor is checked in the presence of Zn (II), Cd (II) and Hg (II) ions. Response of this fluorescent probe is weak due to the absence of protection of the ligand that leads to the weak coordination with Cu (II) ion. The limit of detection of Cu (II) ion is 50 ppb and the maximum limit of presence of this ion is 1.3 ppm. This is the reason it is an effective sensor to check the level of mercury ion in the drinking water. The interaction between Cu (II) ion and DDT.AuNC is oxidative in nature that makes it very specific to copper (II) ion detection [Fig. 2~6].

2.2. Polycaprolactone

Senthamizhan group replaced DTT.AuNC@pCAF fluorescent probe with gold nanocluster (AuNC) for the sensing of Hg (II) ion [22]. The support for the probe is changed from porous cellulose acetate fiber to polycaprolactone nanofiber. The immobilized optical probe on the solid support enhances the lifetime of the sensor to over four months under room condition. Reducing the amount of bovine serum albumin (BSA) ligand from an excessive level to minimum improves the contact of AuNC fluorescent probe Hg (II) ion. It is one of the reasons for the high sensitivity of the probe towards Hg (II) ion even in the presence of Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Mn²⁺ ions. Another reason for the high sensitivity is the formation of amalgam between gold and mercury. It detection level of Hg (II) ion increased to parts per trillion (ppt) level [Fig.7~9].

To create a user-friendly, time-efficient and inexpensive method for Hg²⁺ detection, Tonsomboon et al. developed a cellulose acetate (CA)/polycaprolactone (PCL) membrane-based sensor that utilizes NF06, a novel organic dye, for a colorimetric detection of Hg²⁺ in water samples [23]. The membrane-based sensor was made by immobilizing NF06 onto the electrospun CA/PCL membrane through performing immersion methods twice. Composed of the combination of 2 fluorophores, Helicene (5H) and rhodamine 6 G thiohydrazide (thio-R6 GH), NF06 exhibits a remarkable sensitivity for detecting trace mercury ions. The membrane- based sensor that utilizes this novel optical probe has a limit of detection of 0.309 ppb (0.697 nM), suggesting that this sensor could potentially be used for various environmental and safety-related applications, including the safety evaluation of Hg²⁺ concentration in drinking water. With a facile synthesis method, cost-efficiency and excellent sensitivity, membrane-based sensor that utilizes NF06 shows great promises to enhance the current procedures for detecting Hg²⁺ in various water sources or consumer products.

2.3. Polyvinylalcohol

Using gluten, a plant protein, Mathew et al. developed a novel optical probe for the sensing of Hg²⁺ ions [24]. Processing gluten using hydrothermal carbonisation, graphene quantum dots (GQD) that emits blue-green light was successfully synthesized. Further treatment of GQD with gluten led to the synthesis of gold quantum clusters (AuQC) and incorporating this AuQC@GQD into electrospun polyvinyl alcohol (PVA) nanofibers finished the formation of a nanocomposite sensor for the colorimetric detection of Hg²⁺. The developed sensor showed a sensitivity ranging from 0.1 to 35.8 ppm and the facile detection method, in which it does not require any additional equipment upon determination of Hg²⁺ ions and allows an efficient on-site detection. Thus, the novel AuQC@GQD studied in this research is a green, facile and cost-efficient method for Hg²⁺ detection.

For an easy and fast on-site detection of Hg⁺ ions in real environmental sites, Senthamizhan et al. developed a nanocomposite membrane-based sensor that guarantees a decent sensitivity and an excellent durability in various conditions [25]. The biosensor was created by immobilizing gold nanoclusters (AuNC) in to electrospun polyvinyl alcohol nanofibers (NFM). AuNC*NFM exhibits a noticeably flexibility as well as durability, whose ability to perform fluorescent detection of Hg⁺ remains unaffected up to 100°C. The developed biosensor also has long-lasting storability. With its characteristic color upon contacting Hg⁺ ions, AuNC*NFM allows an easy, naked-eye colorimetric detection without requiring any further equipment, and shows a selective colorimetric change only to Hg⁺ when tested with other interfering ions. Lastly, AuNC*NFM probe was found to have a limit of detection around 1 ppb, and it is encouraged to continue further research to improve its sensitivity to be used as a primary biosensor for on-site detection of Hg⁺ in environmental water.

2.4. Chitosan

In this research, the benefits of using rhodamine B-chitosan moiety, which exhibits fluorescent properties that can be utilized for detection of Hg²⁺, was investigated by Horzum et al. [26]. Developed by the electrospinning of chitosan and rhodamine B hydrazide, the resulting electrospun fiber resulted in biodegradable, high-surface area sensor with enhanced fluorescence that can effectively detect Hg²⁺. Having a sensitivity as low as 10^-9 mM when tested with water samples, the developed sensor also can be used in an on-off manner by using the spirolactam ring of rhodamine for the selective sensing of Hg²⁺. Thus, the developed sensor was posed as an excellent candidate, with environmentallyfriendly synthesis method, sensitivity, and practicability, for Hg²⁺ detection in environmental water.

2.5. Polysulfone

The excessive concentration of Hg⁺ ions in water has become one of the biggest environmental problems globally [27]. To create a tool that could contribute to alleviating this problem, Mukhopadhyay et al. developed a membrane-based sensor for the colorimetric detection of Hg⁺. Made by immobilizing Rhodamine B, a commonly used optical probe, on Polysulfone (PS) and sulfonated Polysulfone (SPS) membranes, the developed sensor exhibits a concentration-dependent sensitivity towards Hg⁺ ions, where increasing the concentration of Hg⁺ in water samples also increased the intensity of color change when tested with a range of Hg²⁺ concentration from 100 ppb to 100ppm. Having a facile synthesis method, where Rhodamine B was attached to membranes through a simple immersion method, and a fast naked-eye colorimetric sensing of Hg²⁺, the developed sensor works as a great candidate for detecting and quantifying the Hg²⁺ concentration in environmental settings.

2.6. Alternative

Upon contacting Ag⁺/Hg²⁺ ions, zein-protected fluorescent gold nanoclusters (Z-FGCs), the novel optical probe studied in this research, exhibits a notable change in its emission at 655 nm, while leaving the emission at 442 nm unaffected [28]. Utilizing this characteristics of zein, Duan et al. proposes the use of Z-FGCs as a new optical probe for detecting Ag⁺/Hg²⁺ ions. Fluorescent gold nanoclusters (FGCs) are generally created by reducing Au (III) in the presence of protecting ligand. In this research, Duan et al. used zein, a plant protein originating from corns, as the protecting and reducing agent when synthesizing FGCs and investigated the benefits of using zein in such procedure. Aside from the environmental-friendly, fast, and cost-efficient synthesis method, using zein also eases the separation of desired Z-FGCs from other, spectator molecules like HAuCl₄ and NaOH, where Z-FGCs can easily be separated by adding acidic buffer, due to the inherent solubility of zein. Also, as zein adds the ability to form films to FGCs, the detection of Ag⁺/Hg²⁺ using FGCs could be simply visualized using Z-FGCs films, without requiring any further, complicated equipment or procedure. Therefore, on top of the high sensitivity of Z-FGCs as optical probes, using zein also poses multiple advantages that simplifies various aspects of Ag⁺/Hg²⁺ detection in water samples.

To form an effective and cost-efficient π-conjugated metal organic framework (MOFs) for a photoluminescence sensing of Hg²⁺, Razavi et al. synthesized a MOF with a structure of [Zn(OBA)-(DPT)_0.5] using a tetrazine-functionalized spacer (TMU-34(-2H)) [29]. By putting the cation detection ability of TMU-34(-2H) into experiment, it was found that TMU-34(-2H) had the ability to accurately sense Hg²⁺ ions in both water and acetonitrile. Using double solvent sensing method (DSSM), in which the signal transduction curves of TMU-34(-2H) in water and acetonitrile were integrated to provide an enhanced sensitivity factor, could upgrade the performance of TMU-34(-2H) to demonstrate an ultrahigh sensitivity. Employing DSSM almost entirely eliminates any interfering, irrelevant responses while showing a higher sensitivity for Hg²⁺ ions. With a short detection time, excellent performance and selectivity, TMU-34(-2H) enables a superb optical detection of Hg²⁺ in various solutions [Fig. 10~12].

2.6.1. MOF

In this research, Wang et al. synthesized two metal organic frameworks (MOFs), whose sensing abilities were tested with various metals [30]. Both MOFs were hydrothermally created using BIPA ligand and the H₂tfbdc ligand with Zn/Cd salts, the first MOF has the structure of [Zn(BIPA)(tfbdc)]_n while the second MOF was found to be {Cd(BIPA)(tfbdc)(H₂O)·DMF}_n. Testing the synthesized MOFs with water samples that contain various metal ions proved that the first MOF, [Zn(BIPA)(tfbdc)]_n, acted as a dual-responsive sensor that effectively detects both Fe³⁺ and Cr₂O₇^2- ions, while the second MOF, {Cd(BIPA)(tfbdc)(H₂O)·DMF}_n showed a selective sensitivity towards Hg²⁺ and Cr₂O₇^2- ions.

Yang et al. constructed a novel porphyrin-based luminescent metal-organic framework (LMOF) for the trace detection of Hg²⁺ ions in water [31]. The LMOF was synthesized by employing solvothermal reaction using Zr-O clusters and meso-tetra(4-carboxyphenyl) porphyrin (TCPP) ligands. The developed MOFs-based sensor showed a high fluorescent ability for detecting Hg²⁺ ions in a concentration-dependent manner, with a lowest limit of detection of 6 nM. Moreover, it guarantees a cost-efficient and fast – where the detection time was found to be around 2 minutes – sensing of the target metal ion in water samples. Demonstrating an intact performance in the presence of other interfering ions, the research poses the advantages of developed and utilizing porphyrin- based sensor for practical application of Hg²⁺ detection.

2.6.2. Polymer membrane

To construct an optical membrane for the detection of heavy metals, Kaoutit et al. utilized copolymers as the sensing platform for a colorimetric detection of Hg²⁺ [32]. The colorimetric assay of Hg²⁺ was simply done by immersing the membranes into water samples. The membranes revealed to display a concentrationdependent behaviour, where the intensity of color change varied depending on the concentration of Hg²⁺ in samples, and the lowest limit of detection was determined to be around 2 ppb (1 × 10^-8 M). Analyzing digital pictures of the membranes also allowed for an approximation of the Hg²⁺ concentration in samples, which implied that the polymer-membrane investigated in this research could be applied for both detection and quantification of Hg²⁺. The application range of such versatile sensor, whose purpose is not only limited to determination of target metal and provides a fast and facile detection method, is wide, and it is found to be especially useful for safety test by measuring the concentration of Hg²⁺ ions in drinking water.

For a colorimetric detection of target metal ions using cycloruthenated complex, Wu et al. used a membranebased sensor that utilizes cyclometallated ruthenium complex (Ru1) as a sensing probe [33]. Exhibiting a color change from dark-red to yellow, the developed sensor was found to have a limit of detection as low as 0.053 μM in aqueous solution. Furthermore, applying the developed membrane-based cycloruthenated complex sensor into water samples that contain Hg²⁺ ions not only allowed for a colorimetric detection, but also removed a noticeable amount of Hg²⁺ ions in water solutions as the Hg²⁺ ions adsorbed onto the surface of the sensor. Thus, the sensor developed and investigated in this sensor shows potential to serve as a dual-purposed sensor for both the detection and filtration of Hg²⁺ contaminations in environmental fields.

3. Conclusions

Gold nanoparticle used for detection are not stable for long time along as well as problem of agglomeration. Alternative to this problem is to immobilization of nanomaterials on the solid substrate. Various kind of support used for this purpose are porous wood, glass and cellulose paper

Nanofiber membranes are excellent support due to high surface area. Synthetic polymer or biopolymers having plenty of functional groups are used for the fabrication of nanofiber membrane which can be very linked to various active molecules. In this review nanofiber membrane made up from cellulose, chitosan, polcaprolactone and polysufone are discussed. Nanofiber membrane with large surface area, good porosity and higher functional group enhance the ability of binding to ligands. Solid state sensor based on immobilized nanoparticles onto nanfiber membrane are much more stable and easily to handle then solution state sensors.

Figures

Fig. 1.

Schematic presentation of classification of the review.

Fig. 2.

(a) SEM images of the electrospun pCAF before DTT.AuNC incorporation. (b) SEM image after incorporating DTT.AuNC. (c) Photograph of the DTT.AuNC@pCAF under UV light (λ_ext-254 nm) and (d) day light condition. TEM image of single DTT.AuNC@pCAF (e) in the presence and (f) absence of excess DTT ligand. (g) Emission spectra of DTT.AuNC solution and DTT.AuNC@pCAF are compared. (Reproduced with permission from Senthamizhan et al., 21, Copyright 2015, Springer Nature).

Fig. 3.

(a) SEM image of DTT.AuNC@pCAF and their corresponding (b–e) elemental mapping images of carbon, oxygen, gold and sulfur, respectively. (f) Image of all elements are overlapped (Reproduced with permission from Senthamizhan et al., 21, Copyright 2015, Springer Nature).

Fig. 4.

(a) Visual colorimetric detection of Cu²⁺. The photograph has been taken under exposure of UV light (λ ext-254 nm). (b) Exposure of DTT.AuNC@pCAF to metal ions and measurement of their fluorescence spectra (c) relative fluorescence intensity variation (Reproduced with permission from Senthamizhan et al., 21, Copyright 2015, Springer Nature).

Fig. 5.

Selective visual colorimetric response of Cu²⁺. (a) Photograph of different metal ion (20 ppm) treatment of DTT.AuNC@pCAF and their (b) corresponding emission spectra. (c) Plot of I₀/I against Cu²⁺ with the competent metal ions (Reproduced with permission from Senthamizhan et al., 21, Copyright 2015, Springer Nature).

Fig. 6.

(a–c) HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) image of Cu²⁺ treated DTT.AuNC@pCAF. STEM-EDX elemental mapping of (d) Au and (e) Cu of formed Au–Cu blend. (f) Mapping images of d and e are overlaped. (g) HRTEM image of Au–Cu blend (Reproduced with permission from Senthamizhan et al., 21, Copyright 2015, Springer Nature).

Fig. 7.

SEM images of the electrospun PCL-NF (a) gold nanocluster (AuNC) coated PCL-NF termed as AuNC*PCL-NF, in the presence (b) and absence (c) of excess BSA ligand. (d) HAADF-STEM elemental mapping of the C, O, S and Au elements present in the AuNC*PCL single NF, shows AuNC are consistently anchored on the surface of NF. (e) Fluorescence image of AuNC*PCL-NF by CLSM, excited at 488 nm. (f) CLSM image of the AuNC*PCL single NF (Reproduced with permission from Senthamizhan et al., 22, Copyright 2015, Springer Nature).

Fig. 8.

(a) CLSM images of AuNC*PCL-SNF before and after addition of Hg²⁺ solution and their corresponding DIC image is presented in Figure. (b) There is no change in their morphology and only the fluorescence feature of the NF changed according to the concentration. (c) Fluorescence spectra taken from the surface of the NF (Reproduced with permission from Senthamizhan et al., 22, Copyright 2015, Springer Nature).

Fig. 9.

Fig. 10.

Fig. 11.

Fig. 12.

Tables

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