1. Introduction

As pollution and contaminations in the natural environment have been becoming one of the most concerning issues in twenty-first century, detection of toxic molecules has been greatly researched and advanced in scientific communities. Among numerous detection methods, colorimetric detection of chemical and biological molecules has been widely studied in recent years due to the advantages it brings to the table. Unlike other commonly used methods, like inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS), colorimetric detections are facile, mostly done without requiring trained personnel or sophisticated equipment[1-10]. Also, most colorimetric sensors perform through simple immersion or dipping methods, which ease the need for complex sample preparation steps and produce results at a substantially lower cost. As the sensing platform, membrane substrates offer benefits due to their porous surface. With such porosity and the presence of abundant functional groups on membrane surface, optical probes can easily immobilize and conduct colorimetric detection on membrane surface. Furthermore, these physical and chemical properties of membranes are tunable during synthesis of membranes, which allow the creation of sensory membranes for specific model analytes[11-20].

In this work, we have summarized the recent works in membrane-based sensors that perform through colorimetric detection. The components of sensors - cellulose membranes and optical reagents - were created and assembled in a facile, simple processes and performed detection of desired molecules without requiring complicated equipment in all of the researches. The porous morphology and chemical features of cellulose membranes, which can be tuned during synthesis process, guarantee a high sensitivity and selectivity of detections. Furthermore, some of the sensors obtained features like antimicrobial ability and reusability, which made these sensors more durable and cost-efficient. Most of the works demonstrate a trace detection of heavy metals in drinking water and environmental water samples, but due to the excessive use of antibiotics prompted by commercial benefits, detection of toxic antibiotics was also covered in this summary. Lastly, study of color response was observed through several methods, such as naked-eye detection, fluorescence intensity, UV-vis spectroscopy and comparison of RGB values before and after experiments. The membrane colorimetric scheme is explained in Fig. 1 and the review is summarized in Table 1.

2. Membrane based Colorimetric Sensor

2.1. Cellulose

Investigating the influence of membrane morphology, especially their porosity, on the functionality of analytic devices, Azmi et al. developed membrane-based colorimetric sensors which demonstrated the correlation between membrane characteristic and quality of colorimetric detection[21] (Figs. 2, 3). The sensors were developed by immobilizing colorimetric probes, which was dithizone (DTZ) in this work, on membrane surface through hydrogen bonding. To study the effect of membrane structure, various types of cellulose acetic membranes (CA) were treated using different polymer- dissolving solvents (formic acid and acetic acid) and non-solvent (DI water). The structure and porosity of membranes varied, and among them, CA membrane treated with formic acid (FA) and DI water (CA-FA : 30DI-DTZ) exhibited the best performance. As for the mechanism of colorimetric detection, the interaction between DTZ and Hg²⁺ ions turned the color of the membrane surface from green to magenta. The color change was observable to the naked eyes down to concentration of 5 ppm for all the sensors developed in this research, but for CA-FA : 30DI-DTZ, the LOD was as low as 3 ppm, exhibiting a superior sensitivity. Proving how a porous membrane surface leads to effective loading of colorimetric probes and thereby high-quality detection, the performance of the developed membrane- based sensors in this research states that the improvement of metal detection can be achieved by tuning the characteristics of membrane structure and chemical properties.

Investigating the influence of the binding mechanism between optical probe and membranes on the colorimetric detection using composite membrane structure, Azmi et al. fabricated a cellulose-based sensor for identifying heavy metals in water[22]. Herein, it was reported that the sensor was built by embedding dithizone (DTZ), the optical probe, onto cellulose acetate membranes. Different concentrations of acetate and chitosan, the key materials in synthesizing cellulose acetate membranes, was used to find the best ratio for optimal immobilization of DTZ on cellulose acetate membranes. It was experimentally found that the ratio for creating a membrane with highest binding ability was using 1 wt.% chitosan and 17 wt.% cellulose acetate. The DTZ-embedded membrane sensors also exhibit a useful stability, where it can retain its functionality up to 90 days. With a high surface area provided by the porosity of the cellulose membranes, the detection of heavy metals was initiated by the adsorption of metal ions on membrane surface, which changed the color of the DTZ through interaction with metal ions. Such color changes occurred rapidly, and the results were visible to the naked eyes. Thus, DTZ-membrane sensors serve as an excellent candidate for facile, rapid, and on-site detection of metal ion concentrations in substances like drinking water and wastewater.

To realize a more efficient detection of trace Cu²⁺ ions in aqueous solution, Feng et al. fabricated a sensory membrane, whose uptake efficiency of Cu²⁺ ions was improved by incorporating filtration method[23]. The sensory membrane was synthesized by grafting diphenylcarbazide (DPC) immobilized sol-gel matrices onto cellulose acetate/nitrate membrane. This composite membrane was then placed in a flow cell connected to a peristaltic pump. The pump pushed the sample solutions into the flow cell, and the Cu²⁺ ions were filtered by the DPC-coated sol-gel membrane due to the chelating interaction between DPC and Cu²⁺ ions. As a result of the chelation, DPC showed a rapid color change from colorless to purple color, where the color intensities were studied by comparing the RGB values in each trial. The developed sensory membrane possessed a high sensitivity with a LOD of 0.16 μM and allowed a quantification of Cu²⁺ ions in the range of 0.16~1.6 μM. Also, the sensitivity of the membrane was unaffected under the influence of other, interfering metals, indicating a good selectivity. In comparison with common immersion or dipping methods, employing peristaltic pump and filtration method increases the chelating interaction between DPC and Cu²⁺ ions, leading to more efficient detection. Thus, a convenient, sensitive and selective determination and quantification of Cu²⁺ ions based on colorimetric response can be achieved with the cellulose acetate/nitrate membrane covered with DPC immobilized sol-gel matrices.

An efficient optical sensor was fabricated by immobilizing chromophore, 2-(2benzothiazolylazo)phenol (BTAP) in this experiment, onto plasticized cellulose triacetate membrane[24]. BTAP was used as its behaviour provided an excellent sensitivity and selectivity for uranium(IV). The optode showed a color change from orange to pink upon contacting uranium - due to the formation of U(VI)-BTAP complex - in bicarbonate/carbonate medium when TEA buffer was also present. All trials in this experiment was conducted at a relatively neutral pH of 6.5~7.0. One obstacle in this mechanism was that BTAP alone could not interact with and capture U(VI) from the aqueous solution. Thus, DNNS was selected as the carrier for transferring U(VI) from aqueous solution to optode matrix, increasing the efficiency of U(VI) uptake and facilitating the color response. The intensity of the color change correlated to the uranium concentration, thus quantification of uranium ions. The optode also showed a good selectivity, where the presence of other interfering ions did not impactfully affect the sensitivity of the optode as long as they did not exceed micro-molar concentrations. Various types and concentrations of carriers and plasticiers were used to minimize the time needed for the optode to display a full color response, and it was found that the following conditions optimized the performance of optode: 24 wt% cellulose triacetate matrix, 35 wt% DNNS, 40 wt% TEHP, 0.1 wt% BTAP. The optode produced satisfactory results when tested with real samples and was faster, simpler than conventional methods like graphite furnace atomic absorption spectroscopy, which showed its potential to be applied for detection of radionuclides in the environment.

Using a simple, green, and inexpensive approach, Jiang et al. fabricated a cellulose membrane based colorimetric sensor strip (CCSS)[25] (Figs. 4~7). CCSS was created by immobilizing Victoria blue B (VBB) on the surface of cellulose membranes through hydrogen bonding between VBB and hydroxyl groups on the cellulose membranes. Various techniques, such as SEM, FTIR and BET, were used to confirm the successful fabrication of CCSS. With such characterizations, it was found that the porosity and surface area of cellulose membranes decreased after adding VBB. When Cd(II) was present in water samples, CCSS showed a rapid color change from yellow to blue-green within 1 minute. The color change was obvious enough to be observed by naked eyes - with the LOD of 0.01 mg L^-1 - and the intensity of change correlated to the concentration of Cd(II). The selectivity of CCSS were also found to be excellent, as the membranes showed almost no change when interfering metals were added to the samples. Having great advantages like high sensitivity, time-efficiency and selectivity, the fabricated CCSS were able to detect cadmium ions in local lake water. Thus, CCSS demonstrates qualities that make it a potent candidate as a portable and sensitive device for a rapid detection of contaminants, especially heavy metals in real water samples.

The sensor developed by Lee et al. and discussed in this research is a paper-based membrane whose colorimetric detection was enabled by the adsorption of gold nanoparticles (Au NPs)[26] (Fig. 8). To create the sensor, Au NPs functionalized with bovine serum albumin (BSA) where embedded on the surface of nitrocellulose membrane (NCM), creating a nanostructure called BSA-AuNPs/NCM. The detection method in this sensor utilizes the interaction between Pb²⁺ ions and AU NPs, where Pb²⁺ promotes the leaching of AuNPs when reagents like thiosulfate (S₂O₃^2-) and 2-mercaptoethanol (2-ME) were also present. Thus, detection of Pb²⁺ is carried out by the color changes that accompanies when Au NPs are detached from the membrane surface. Through experimentation of various conditions, it was found that when an environment of 5 nM of glycine- NaOH, 10 mM of S₂O₃^2-, and 250 mM of 2-ME was achieved, with the addition of using microwave irradiation at 450W for decreasing the reaction time, the most superior result was achieved. In this best result, the LOD for the detection of Pb²⁺ was 50 pM with an excellent selectivity. Additionally, the sensor was successful in testing the presence of Pb²⁺ ions in real samples like seawater, urine and blood, which guaranteed the practicability of this simple, selective and inexpensive membrane-based sensor.

Introducing a simple, unique and time-efficient method for the pre-concentration of sample analyte, Alahmad et al. developed a μPADs for the colorimetric detection of hexavalent chromium[27]. Creating a μPADs as an platform for colorimetric detection, hollow fiber membrane liquid phase microextraction (HF-LPME) was employed to preconcentrate hexavalent chromium, which led to a more effective detection. Coupling HF-LPME not only guaranteed a sensitive determination through pre-concentration of analyte but also reduced the cost and time required for fabricating sensors and preparing sample solutions. The optical probe selected for the determination of analyte was diphenylcarbazide (DPC), which is known to show a color response of forming purple complexes upon contacting hexavalent chromium. For a sophisticated investigation of the sensitivity and performance of developed μPADs, the images of sensors were taken and their color intensities, more specifically the green color in the RGB method, were studied. When the sensitivity of the μPADs was tested with sample solutions and real samples, the LOD was found to be as low as 3 μg L^-1, and the color intensities showed a concentration-dependent behaviour. In comparison with the results obtained from ICP-AES, the analysis of real samples using the developed μPADs seemed to hold a high accuracy. The integration of techniques introduced in this report can be used for a facile, cost-efficient and accurate detection of hexavalent chromium in various water samples.

Abedalwafa et al. developed a novel sensory membrane for the detection of oxytetracycline[28] (Figs. 9~ 11). Oxytetracycline (OTC), a commonly used antibiotics that could cause deteriorating effects when it enters human body. The sensory membrane for targeting this antibiotic was fabricated by embedding nickel ions on carboxymethylcellulose/polycrylonitrile nanofibrous membranes (Ni@CMC/PAN NFMs). In the presence of Ni²⁺, the color of the sensory membranes quickly changes from light-green to yellow due to the formation of Ni-OTC complex. Various conditions like pH, temperature, and the concentrations of chemical reagents were experimented to optimize features like pore size, porosity, and specific surface area, which could improve the binding efficiency of Ni²⁺ and OTC to improve the sensitivity of Ni@CMC/PAN NFMs. The sensitivity achieved through this process was excellent, with the lowest limit of 5 nM for the colorimetric detection by naked eyes. Ni@CMC/PAN NFMs also possessed a good selectivity, where it did not exhibit any significant color change in the presence of antibiotics other than OTC. Also, the chemical complex Ni-OTC could be chemically removed after detection, which indicated that Ni@CMC/PAN NFMs can be reused for future purposes when it goes through cleaning cycles. Demonstrating a high sensitivity, selectivity, and durability, Ni@CMC/PAN NFMs can provide a fast, facile colorimetric detection of antibiotics that are observable by naked eyes.

2.2. Polyvinyl chloride

To create a novel μPADs that overcomes the current limitations, Sharifi et al. developed a 3D origami paper- based device and used it for examining a real sample[ 29]. The developed μPADs had multi-layered structure including sample loading layer and detection layer. When characterizing the μPADs, it was found that adding a third layer, acting as a waste site, could increase the volume of sample solution added without overflowing. Commonly, the movement of colored reagents when using conventional μPADs often led to the inconsistent distribution of reagents on the surface of the paper, which limited the sensitivity and lowering of detection limits. Thus, incorporating polyvinyl chloride (PVC) membrane into μPADs anchored the optical reagents on paper, which prevented the loss of dye and color heterogeneity on paper surface when sample solution was added. Such modification improved the sensitivity and accuracy of analyte detection. To test the practicability of the novel μPADs, the detection of Cu(II) ions was done in real rain water and tap water. Using two optical reagents, pyrocatechol violet and chrome azurol S, the μPADs was found to hold a high sensitivity, with LOD of 1.9 mg L^-1 and 1.7 mg L^-1 for each reagent respectively. As on-site regulations of heavy metal ions are becoming a serious environmental topic, the rapid, facile, and sensitive μPADs can bring advances to the current methods for monitoring and managing the level of contaminants in natural environments.

2.3. Polyacrylonitrile

For the creation of a stable, durable sensor for the colorimetric detection of Hg²⁺, Senthamizhan et al. developed a nanostructure involving electrospun nanofibrous membrane (NFM) and gold nanoclusters (AuNC)[30]. The fabrication of this nanostructure was done by embedding synthesized AuNC onto electrospun polyvinyl alcohol nanofibers, which was then cross-linked with glutaraldehyde vapor for stronger durability. When this sensor, called AuNC*NFM, was immersed in a solution of Hg²⁺, it demonstrated a clear colorimetric detection as its color changed from red to dark blue under UV light. The accuracy and selectivity, where it did not respond to other common contaminating metals in drinking water, was confirmed by fluorescence test, where contacting with Hg²⁺ impactfully decreased the intensity of fluorescence emitted from AuNC*NFM. Moreover, AuNC*NFM was confirmed to possess stability over an extended period of time and endure temperature up to 100°C without suffering from structural or functional deterioration, which posed it as an excellent sensor for a wide range of settings. Allowing naked-eye detection under UV light, displaying selectivity, facile fabrication methods and prolonged usability, AuNC*NFM entails potency for on-site, colorimetric detection and monitoring of mercury levels in various environments.

2.4. Others

Investigating the use of copolymers as sensing platforms, Kaoutit et al. reported the development of sensory membranes for the regulation of Hg²⁺ concentration in water samples[31]. When the membranes were dipped in water samples with ranging concentrations of Hg²⁺, the intensity of color change varied depending on the concentration of Hg²⁺. Such result confirmed that the developed membranes not only accurately detected the presence of Hg²⁺, but could also enable quantification of probe molecule by analyzing pictures of the membranes taken with conventional cameras. The sensitivity of these membranes was also found to be excellent, as they exhibited the ability to detect Hg²⁺ even in nanomolar ranges. Thus, the sensory membranes were found useful in various applications, such as detecting the level of Hg²⁺ contamination in water samples and was found especially applicable for determining whether a sample exceeds the permittable Hg²⁺ concentration for drinking water.

For an efficient determination of Hg²⁺ ions in water samples, Mukhopadhyay created polymer membranes covered with colorimetric reagents and reported their functionalities in this research[32]. Selecting Rhodamine B as the colorimetric reagent, Rhodamine B was immobilized on polysulfone membrane and sulfonated poly sulfone membrane. By simply immersing the sensory membranes in sample solutions, a rapid color change and determination of Hg²⁺ ions were achieved. Also, the intensity of color change linearly correlated with the concentration of Hg²⁺ ions in solutions, enabling a quantification of Hg²⁺ ions. By comparing the binding efficiency of Rhodamine B and Hg²⁺ ions on polysulfone membrane and sulfonated polysulfone membrane, the influence of physical features, like porosity, and functional groups of membrane substrates on the binding of chemical reagents were examined. With the accumulation of pollutants in the natural environment, heavy metals, especially Hg²⁺, is creating environmental degradations in a rapid rate. It is crucial to detect and regulate Hg²⁺ level in these environments, and the portable sensor membranes developed in this research can provide a rapid, simple detection and quantification of Hg²⁺ ions.

In this report, Wang et al. developed a colorimetric probe for the determination of arsenic(V) in sample solutions[33]. The novel probe was synthesized by modifying FeOOH with L-arginine (2Arg@FeOOH) through a facile method. This probe showed a behaviour similar to peroxidase, breaking down H₂O₂ and forming hydroxyl groups that oxidized TMB, the colorimetric dye selected in this experiment. Such chemical reaction turned the colorless TMB into blue color. The determination of As(V) utilizes this chemical reaction; upon contacting As(V), the peroxidase-like activity of 2Arg@FeOOH terminates and notifies the presence of As(V). Having such ability to interact with As(V), 2Arg@FeOOH was further modified to be mounted onto fiber membrane (FM), forming 2Arg@FeOOH/FM, which allowed the removal of As(V) from sample solutions. When the removal efficiency of 2Arg@FeOOH/ FM was experimented in static removal and dynamic removal (employing circulating filtration) methods, the maximum efficiency was found to be as high as 95% (in dynamic removal). The sensitivity of the colorimetric detection was also excellent, with LOD of 0.42 μg L^-1 and a wide detection range from 0.67 μg L^-1 to 3333.33 μg L^-1. Lastly, the 2Arg@FeOOH/FM also provided antimicrobial activity, which can increase the durability of sensor when treating wastewater as it would prevent the accumulation of bacteria on membrane surface. Overall, 2Arg@FeOOH/FM serves as an ideal candidate for the determination and filtration of pollutants in environmental water.

Demonstrating the use of cycloruthenated complex for heavy metal detection, Wu et al. developed a dye and employed for detecting Hg²⁺ ions in sample solution[ 34]. When the novel dye, called cyclometallated ruthenium complex (Ru1), was synthesized and tested, a strong color change from dark-red to yellow occurred and was observable by naked eyes. It was found that the Ru1 exhibited a high sensitivity, where the LOD was around 0.053 μM in aqueous solution. Moreover, it was found that the complex was only responsive to mercury ions even when other, commonly found metal ions existed, possessing a high selectivity that remains intact in the presence of interfering ions. Ru1 was then mounted on the membrane, which provided a solid, stable platform for metal detection to take place. When Ru1 was present alone, without a membrane substrate, it could only serve as a dye that exhibited color change when it contacted Hg²⁺. But, when Ru1 was grafted onto membrane, the Hg²⁺ ions adsorbed on the membrane surface upon reacting with Ru1, removing a large portion of Hg²⁺ ions from the sample solution. Thus, when membrane was incorporated, detection as well as removal of Hg²⁺ ions were feasible. As mercury is one of the most toxic metals that accumulates in the environment and threatens animal and human health, regulation and removal of mercury pollution is crucial. The novel dye Ru1 and its incorporation into a membrane substrate show potency for a more effective detection and filtration of Hg²⁺ in the environmental systems.

Ghobashy et al. reported the fabrication of a composite nanostructure, in which copper nanoparticles were capped with polymer membranes for the colorimetric detection of Hg²⁺ and Ag⁺[35]. For the creation of this nanostructure, synthesized copper NPs were firstly reduced with ascorbic acid. Subsequently, the solution with reduced copper NPs were added to a membrane syn thesized using polyvinyl alcohol (PVA) and acrylic acid (AAC), leading to the development of a nanostructure called Cu-(PAAc/PVA). The synthesis of nanostructure was facilitated by gamma radiation. When each analyte was tested with Cu-(PAAc/PVA), the silver ions were reduced by copper ions in the membrane and mercury ions formed amalgam on the membrane surface. The corresponding Cu-(PAAc/PVA) color changes were from yellow to dark green and from yellow to white for silver ions and mercury ions, respectively. These color changes were monitored using UV-vis spectroscopy as change in the color of membranes were accompanied by absorption change in UV-vis spectrum and indicated that the LOD were as low as 10^-5 M (mercury) and 10^-6 M (silver). As heavy metals are posing threats to environmental systems and human health, causing various pollutions and health issues when accumulated, the nanostructure developed in this research can provide a rapid, simple, cost-efficient and sensitive detection of Hg²⁺ and Ag⁺, allowing an efficient monitoring of the level of toxic metals.

Zhang et al. reported the fabrication of a reusable, colorimetric sensor for the detection of Cu²⁺ and Fe³⁺ in aqueous solutions[36] (Figs. 12~15). For the sensor, poly(aspartic acid) (PASP) was incorporated into electrospun nanofiber hydrogel membrane (ENHM) through electrospinning and further modifications. Through this configuration, the sensor benefited from both characteristics of PASP and ENHM, where PASP facilitates ion adsorption as it easily binds to metals and ENHM provides a large specific surface area. As a result, the sensor was capable of separating and detecting Cu²⁺ and Fe³⁺ by capturing the metal ions on the surface of the sensor when sample solution was filtered through. With the accumulation of the adsorbed metal ions, the color of the sensors changed from white to blue for the detection of Cu²⁺, and from white to yellow when Fe³⁺ was present. The intensity of the color change also responded to the concentration of the metal ions in the solutions, and the detectable range for the Cu²⁺ and Fe³⁺ ions were 0.3 to 30 mg/L and 0.1 to 10mg/L respectively. The lowest detection limits (0.3 mg/L and 0.1 mg/L) of ions are both within the permissible limit of Cu²⁺ and Fe³⁺ concentrations in drinking water, which makes it an excellent sensor for ensuring the quality of drinking water. Additionally, PASP-ENHM remains relatively inactive to other types metal ions and can detect a mixture of Cu²⁺ and Fe³⁺ through a blend of blue and yellow colors, acting as a sensitive, selective, and reusable sensor for the regulation of drinking water in real application.

For the dual detection of Cu²⁺ and Mn²⁺, Si NPs were synthesized through a simple process of mixing two raw materials, where uric acid acted as the reducing agent, and yielded a solution of desired Si NPs[37]. When Cu²⁺ and Mn²⁺ were added to this as synthesized Si NP solutions, the color of the solution visibly deepened in yellow and the excitation of the mixed solutions at 345 nm revealed a strong, enhanced emission of the Si NPs. Thus, using Si NPs provided both colorimetric and fluorescent detection of Cu²⁺ and Mn²⁺, with a high selectivity - mixing Si NPs with other heavy metals didn’t display any color change or enhanced fluorescence. Furthermore, Si NPs could be rendered into different forms for a wider range of applications. Rendering Si NPs as gel membrane realized fluorescent detection on a different platform, where the fluorescence emission from the membrane intensified upon detecting Cu²⁺ or Mn²⁺. Or Si NPs could be made into powdered form for latent fingerprinting, where Si NP power provided detailed fingerprints of individuals and was used for collection of fingerprints on various types of surfaces and materials. This multiplexed probe, which facilitates various forms of detections - simple colorimetric detections, fluorescent emissions in solutions, gel membrane detection and latent fingerprinting - for both Cu²⁺ and Mn²⁺. Such versatility ensures practical applications for the monitoring and regulation of Cu²⁺ and Mn²⁺ in a wide range of surfaces and environments.

3. Conclusions

Herein, we have classified the researches focused on membrane-based sensors based on the types of membrane used: cellulose, polyvinyl chloride, polyacrylonitrile, and other miscellaneous membrane sensors. Various types of membranes were used by different researchers to suit each research’s purpose of detection. Utilizing the porosity and specific surface area, which make membrane surfaces a superior substrate over other materials, membrane surfaces were functionalized with ideal optical reagents. Many of the works used metal nanoparticles or fluorescent probes as optical reagents, which exhibit a rapid color response visible to naked eyes or a change in absorbance spectrum upon detecting the model analyte. In some researches, fluorescence was used or difference in RGB intensities were measured to perform the colorimetric detection. With excellent sensitivity and selectivity, the detection of model analytes is easily conducted through simple immersion or dipping methods when membrane-based sensors are used. Furthermore, cellulose membranes also possess great mechanical strength and are easy to transport, which make them as ideal sensors for on-site detection in various sites. The versatile, superb membrane-based sensors were applied in a wide range of settings, mainly in detection and quantification of heavy metals and toxic antibiotics in various water samples.