Electrochemical sensor for the detection of estradiol based on electropolymerized molecularly imprinted polythioaniline film with signal amplification using gold nanoparticles

A molecularly imprinted sensor for single-molecule detection of pesticide metabolite at the amol/L level sensitized by water-soluble luminol derivative encapsulated liposome via click reaction

A novel luminol derivative, 4-[(1,4-dioxo-1,2,3,4-tetrahydrophthalazin-5-yl)amino]-4-oxobut-2-enoic acid (ALD) with electrochemiluminescence intensity and stability characteristics similar to luminol, but higher solubility in near neutral solution, was designed and synthesized in this study. Using this derivative, a molecular imprinted electrochemiluminescence sensor (MIECLS) was prepared for the sensitive and selective determination of 2-amino-5-mercapto-1,3,4-thiadiazole (AMT), a metabolite of bismerthiazol, thiediazole copper, thiazole zinc, and other pesticides. The ALD probes encapsulated in liposomes are immobilized on the molecularly imprinted film by light-triggered click reaction, and the concurrent release of multiple probes allows for highly sensitive detection. In the AMT concentration range of 1.00 × 10-18 - 5.00 × 10-13 mol/L, the relation between ECL response and log AMT concentration is linear. With a detection limit of 5.25 × 10-19 mol/L (about 4 - 6 molecules in 10 μL of the sample), the sensor allows for high sensitivity analysis of ultra-trace amounts of small organic compounds. In general, the ECL-based single-molecule detection technique proposed herein might be a promising alternative to fluorescence single-molecule detection.

Sensitive detection of 17β-estradiol at a picomolar level using an aptamer-assisted liquid crystal-based optical sensor

A liquid crystal (LC)-based aptasensor was developed that can detect 17β-estradiol (E2) at the picomolar level. This aptasensor is based on competitive reactions of the aptamer that interacts with cetyl trimethyl ammonium bromide (CTAB) and E2 at the aqueous/LC interface. The long alkyl chain of CTAB anchored the 4-cyano-4'-pentylbiphenyl (5CB) to a homeotropic state and controls the local anchoring depending on the extent of electrostatic interaction with the aptamer. Upon addition of the aptamer solution to the CTAB-saturated LC layer, LCs change from dark to bright optical response. This is due to the perturbed orientation of 5CB at the aqueous/LC interface as a result of electrostatic attraction of the cationic group of CTAB and the phosphate group of the aptamer. The conformational change of the aptamer due to specific binding with E2 weakens the electrostatic attraction between CTAB and aptamer. When specific binding becomes relatively dominant, CTAB induces the orientation of LCs to the homeotropic state, resulting in a dark optical image observed. We also analyzed the change in the optical response of LCs according to the interfacial events and compared the grayscale values of the optical image for each concentration of E2 to determine the detection limit. Accordingly, the detection limit of the E2 sensor was found to be 3.1 pM (0.8 pg/ml) in Tris-buffered saline (TBS), and 6.8 pM (1.9 pg/ml) in human urine. The LC-based optical aptasensor was thus shown to be highly sensitive and selective with no requirement for complex analysis equipment.

Picomolar or beyond Limit of Detection Using Molecularly Imprinted Polymer-Based Electrochemical Sensors: A Review

Over the last decades, molecularly imprinted polymers (MIPs) have emerged as selective synthetic receptors that have a selective binding site for specific analytes/target molecules. MIPs are synthetic analogues to the natural biological antigen-antibody system. Owing to the advantages they exhibit, such as high stability, simple synthetic procedure, and cost-effectiveness, MIPs have been widely used as receptors/sensors for the detection and monitoring of a variety of analytes. Moreover, integrating electrochemical sensors with MIPs offers a promising approach and demonstrates greater potential over traditional MIPs. In this review, we have compiled the methods and techniques for the production of MIP-based electrochemical sensors along with the applications of reported MIP sensors for a variety of analytes. A comprehensive in-depth analysis of recent trends reported on picomolar (pM/10^-12 M)) and beyond picomolar concentration LOD (≥pM) achieved using MIPs sensors is reported. Finally, we discuss the challenges faced and put forward future perspectives along with our conclusion.

Alternative methods of monitoring emerging contaminants in water: a review

Anthropogenic activities have steadily increased the release of emerging contaminants (ECs) in aquatic bodies, and these ECs may have adverse effects on humans even at their trace (μg L^-1) levels. Their occurrence in wastewater systems is more common, and the current wastewater treatment facilities are inefficient in eliminating many of such persistent ECs. "Gold standard" techniques such as chromatography, mass spectrometry, and other high-resolution mass spectrometers are used for the quantification of ECs of various kinds, but they all have significant limitations. This paper reviews the alternative methods for EC detection, which include voltammetry, potentiometry, amperometry, electrochemical impedance spectroscopy (EIS) based electrochemical methods, colorimetry, surface-enhanced Raman spectroscopy (SERS), fluorescence probes, and fluorescence spectroscopy-based optical techniques. These alternative techniques have several advantages over conventional techniques, including low sample volume, excludes solid phase extraction procedure, high sensitivity, selectivity, portability, reproducibility, rapidity, low cost, and the ability to monitor ECs in real time. This review summarises each of the alternative methods for detecting ECs in water samples and their respective limits of detection (LODs). The sensitivity of each technique varied depending on the type of EC measured, type of electrochemical probe and electrode, substrates, type of nanoparticle (NP), the physicochemical parameters of water samples tested, and more. Nevertheless, this paper also focuses on some of the current challenges encountered by these alternative methods in monitoring ECs.

Design and fabrication of electrochemical sensor based on molecularly imprinted polymer loaded onto silver nanoparticles for the detection of 17-β-Estradiol

In this research report, we prepared an electrochemical sensor based on the molecularly imprinted poly(p-aminophenol) supported by silver nanoparticles capped with 2-mercaptobenzoxazole (AgNP) for the selective and sensitive detection of endocrine disrupting 17β-estradiol (E2). The electropolymerization of the functional monomer prepared the proposed MIP composite-based sensor in the presence of E2 as a template. The recognition materials were characterized using Fourier transform infrared, cyclic voltammetry (CV), square wave voltammetry (SWV), scanning electron microscopy, energy-dispersive X-ray spectroscopy and x-ray powder diffraction techniques. The electrochemical measurements were performed by employing both CV and SWV methods. We did the optimization of critical parameters affecting the sensor performances through the experimental design and verification. The developed sensor showed a linear range from 10 pM to 100 nM with the calculated quantification and detection limits of 1.86 pM and 6.19 pM, respectively. The incorporation of AgNP with high electrical conductivity into the MIP matrix enhanced the sensor's performance. Furthermore, the sensor was applied to determine E2 in real water samples without any sample preconcentration steps to achieve the percent recovery of 91.87-98.36% and acceptable reusability and storage stability performances. This article is protected by copyright. All rights reserved.

Highly-sensitive and simple fluorescent aptasensor for 17 b-estradiol detection coupled with HCR-HRP structure

As an important kind of environmental endocrine disruptors, 17 β -Estradiol (E2) plays a major role in affecting the growth of human including sexual characters, pregnancy system, etc. In the modern society, with the threat of abuse in breeding, it is imperative to design sensitive methods for detecting low concentration of E2 in environment. In this work, we constructed a highly sensitive and simple fluorescent aptasenor for detecting E2 via amplification of hybridization chain reaction (HCR) and horseradish peroxidase (HRP). Through the competitions between complementary strand (cmDNA) and E2 to E2 aptamer modified on magnetic beads, the unbound cmDNA would be collected and captured by polystyrene microspheres to induce HCR which brought abundant biotin sites. Subsequently, benefit from the excellent catalytic performance of streptavidin-horseradish peroxidase (SA-HRP), the highly sensitive fluorescence signals could be obtained in low concentration of E2. Under the optimal conditions, the prospered method for E2 detection was shown a good liner range from 1 to 100 pg/mL, with the lower detecting limit of 0.2 pg/mL compared with previous work. In addition, the recovery rates tested in the real samples of milk and water were 99.20%-108.06% and 91.07%-106.13%. In all, the assay may provide a perspective way for highly sensitive detection for various contaminants in the real samples.

A versatile and ultrasensitive molecularly imprinted electrochemiluminescence sensor with HRP-encapsulated liposome labeled by light-triggered click reaction for pesticide residues

A novel approach for trace detection of fipronil with a molecularly imprinted electrochemiluminescence sensor (MIECLS) is proposed. The sensitivity is significantly improved via signal amplification of the enzymatic reaction of horseradish peroxidase (HRP) released from encapsulated liposomes which linked onto the template molecules after rebinding. The molecularly imprinted polymer membrane was prepared through the electropolymerization of monomers with fipronil as a template. After the elution of the template molecules, the analyte fipronil was reabsorbed into the cavities. HRP-encapsulated liposomes were linked to the target molecules by light-triggered click reaction. The higher the concentration of the target was, the more HRP-encapsulated liposomes were present on the molecularly imprinted polymer (MIP) sensor. Then, HRP was liberated from liposomes, and the catalytic degradation of hydrogen peroxide (H₂O₂) by HRP occurs, which changed the electrochemiluminescence intensity of luminol significantly. The change of the ∆ECL intensity was linearly proportional to the logarithm of the fipronil concentration ranging from 1.00 × 10^-14 to 1.00 × 10^-9 mol/L, and the detection limit was 7.77 × 10^-16 mol/L. The recoveries obtained ranged from 95.7 to 105.8% with RSD < 5%. The sensitivity of the detection was significantly improved, and the analysis process was simplified in that the incubation step required in the conventional method was avoided. The sensor proposed provides a feasible platform for ultra-trace amount determination.

Extremely Sensitive Molecularly Imprinted ECL Sensor with Multiple Probes Released from Liposomes Immobilized by a Light-Triggered Click Reaction

A molecularly imprinted electrochemiluminescence sensor was prepared for sensitive and selective determination of aminotriazole via a novel strategy of multiple Ru(bpy)₃Cl₂ probes released from liposomes immobilized by a light-triggered click reaction. This sensing strategy provides a platform for trace detection of amino-containing pesticides. The target on the molecularly imprinted membrane connected to the Ru(bpy)₃Cl₂-encapsulated liposomes via the click reaction. After the destabilizing agent Triton X-100 was added, numerous Ru(bpy)₃Cl₂ molecules were released by liposomes on the molecularly imprinted polymer electrode. The ECL response of the sensor was linearly proportional to the logarithm of the aminotriazole concentration ranging from 5.00 × 10^-18 to 1.00 × 10^-12 mol/L, and the detection limit was 1.15 × 10^-18 mol/L. The sensitivity of the detection was significantly improved, and the analysis process was simplified.

A Review of Nanocomposite-Modified Electrochemical Sensors for Water Quality Monitoring

Electrochemical sensors play a significant role in detecting chemical ions, molecules, and pathogens in water and other applications. These sensors are sensitive, portable, fast, inexpensive, and suitable for online and in-situ measurements compared to other methods. They can provide the detection for any compound that can undergo certain transformations within a potential window. It enables applications in multiple ion detection, mainly since these sensors are primarily non-specific. In this paper, we provide a survey of electrochemical sensors for the detection of water contaminants, i.e., pesticides, nitrate, nitrite, phosphorus, water hardeners, disinfectant, and other emergent contaminants (phenol, estrogen, gallic acid etc.). We focus on the influence of surface modification of the working electrodes by carbon nanomaterials, metallic nanostructures, imprinted polymers and evaluate the corresponding sensing performance. Especially for pesticides, which are challenging and need special care, we highlight biosensors, such as enzymatic sensors, immunobiosensor, aptasensors, and biomimetic sensors. We discuss the sensors' overall performance, especially concerning real-sample performance and the capability for actual field application.

Molecularly imprinted polymers - towards electrochemical sensors and electronic tongues

Molecularly imprinted polymers (MIPs) are artificially synthesized materials to mimic the molecular recognition process of biological macromolecules such as substrate-enzyme or antigen-antibody. The combination of these biomimetic materials with electrochemical techniques has allowed the development of advanced sensing devices, which significantly improve the performance of bare or catalyst-modified sensors, being able to unleash new applications. However, despite the high selectivity that MIPs exhibit, those can still show some cross-response towards other compounds, especially with chemically analogous (bio)molecules. Thus, the combination of MIPs with chemometric methods opens the room for the development of what could be considered a new type of electronic tongues, i.e. sensor array systems,  based on its usage. In this direction, this review provides an overview of the more common synthetic approaches, as well as the strategies that can be used to achieve the integration of MIPs and electrochemical sensors, followed by some recent examples over different areas in order to illustrate the potential of such combination in very diverse applications.

Molecularly imprinted polymer-based electrochemical sensors for environmental analysis

The ever-increasing presence of contaminants in environmental waters is an alarming issue, not only because of their harmful effects in the environment but also because of their risk to human health. Pharmaceuticals and pesticides, among other compounds of daily use, such as personal care products or plasticisers, are being released into water bodies. This release mainly occurs through wastewater since the treatments applied in many wastewater treatment plants are not able to completely remove these substances. Therefore, the analysis of these contaminants is essential but this is difficult due to the great variety of contaminating substances. Facing this analytical challenge, electrochemical sensing based on molecularly imprinted polymers (MIPs) has become an interesting field for environmental monitoring. Benefiting from their superior chemical and physical stability, low-cost production, high selectivity and rapid response, MIPs combined with miniaturized electrochemical transducers offer the possibility to detect target analytes in-situ. In most reports, the construction of these sensors include nanomaterials to improve their analytical characteristics, especially their sensitivity. Moreover, these sensors have been successfully applied in real water samples without the need of laborious pre-treatment steps. This review provides a general overview of electrochemical MIP-based sensors that have been reported for the detection of pharmaceuticals, pesticides, heavy metals and other contaminants in water samples in the past decade. Special attention is given to the construction of the sensors, including different functional monomers, sensing platforms and materials employed to achieve the best sensitivity. Additionally, several parameters, such as the limit of detection, the linear concentration range and the type of water samples that were analysed are compiled.

Design of a ratiometric fluorescence sensor based on metal organic frameworks and Ru(bpy)32+-doped silica composites for 17β-Estradiol detection

17β-Estradiol (E2), an important endocrine disrupting compound, could be quantitatively detected by fluorescence resonance energy transfer (FRET) aptasensor, designed in this paper. Metal organic frameworks have large specific surface area and easily modifiable groups, which are helpful for the construction of aptasensor. Specifically, streptavidin was immobilized on the synthesized MIL-53-NH₂ by covalent bonding, and further linked with the biotin modified E2 aptamer (apt) through specific bonding between avidin and biotin to obtain the FRET donor probe (MIL-53-apt). Meanwhile, complementary DNA (cDNA) modified Ru(bpy)₃²⁺-doped silica nanoparticles (RuSiO₂-cDNA) were prepared through covalent bonding. They acted as the FRET acceptor probe, since its absorption spectrum showed large overlap with the emission spectrum of MIL-53-apt. In the presence of E2, aptamer modified donor probes tended to bind with E2, owing to their higher selectivity and affinity. Therefore, the optimal distance between FRET pairs was broken, resulting in the fluorescence emission recovery of donor and the fluorescence emission of acceptor decreased. Under optimal conditions, this proposed aptasensor displayed sensitive detection of E2 ranging from 0.5 to 1000 nM with a detection limit of 0.2 nM. Furthermore, the sensor provides a promising method for rapid and sensitive detection of other small biological molecules.

Voltammetric sensor based on cobalt-poly(methionine)-modified glassy carbon electrode for determination of estriol hormone in pharmaceuticals and urine

A voltammetric sensor based on the electropolymerization of cobalt-poly(methionine) (Co-poly(Met)) on a glassy carbon electrode (GCE) was developed and applied for the determination of estriol by differential pulse voltammetry (DPV) for the first time. The electrochemical properties of the Co-poly(Met)/GCE were analysed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the polymers on the GCE surface. The deposition of the Co-poly(Met) film on the GCE surface enhanced the sensor electronic transfer. CV studies revealed that estriol exhibits an irreversible oxidation peak at +0.58 V for the Co-poly(Met)/GCE (vs. Ag/AgCl reference electrode) in 0.10 mol/L Britton-Robinson buffer solution (pH = 7.0). Different voltammetric scan rates (10–200 mV/s) suggested that the estriol oxidation on the Co-poly(Met)/GCE surface is controlled by adsorption and diffusion processes. Based on the optimized DPV conditions, the linear responses for estriol quantification were from 0.596 μmol/L to 4.76 μmol/L (R² = 0.996) and from 5.66 μmol/L to 9.90 μmol/L (R² = 0.994) with a limit of detection (LOD) of 0.0340 μmol/L and a limit of quantification (LOQ) of 0.113 μmol/L. The DPV-Co-poly(Met)/GCE method provided good intra-day and inter-day repeatability with RSD values lower than 5%. Also, no interference of real sample matrices was observed on the estriol voltammetric response, making the DPV-Co-poly(Met)/GCE highly selective for estriol. The accuracy test showed that the estriol recovery was in the ranges 96.7%–103% and 98.7%–102% for pharmaceutical tablets and human urine, respectively. The estriol quantification in pharmaceutical tablets performed by the Co-poly(Met)/GCE-assisted DPV method was comparable to the official analytical protocols.

Doping of transition metal dichalcogenides in molecularly imprinted conductive polymers for the ultrasensitive determination of 17β-estradiol in eel serum

Molecularly imprinted polymers (MIPs) have been developed to replace antibodies for the recognition of target molecules (such as antigens), and have been integrated into electrochemical sensing approaches by polymerization onto an electrode. Electrochemical sensing is inexpensive and flexible, and has demonstrated utility in point-of-care devices. In this work, several 2D (conductive) materials were employed to improve the performance of MIP sensors. Screen-printed electrodes were coated by the electropolymerization of aniline and metanilic acid, commingled with target molecules and various 2D materials. Tungsten disulfide (WS₂) with an average particle size of 2 μm was found to increase the sensitivity of detection of molecularly imprinted conductive polymer-coated electrodes to 17β-estradiol. As estradiol concentrations are important to eel aquaculture, we screened eel serum samples to determine their 17β-estradiol concentrations, which were found to be in the range 28.2 ± 3.6 to 73.0 ± 11.6 pg/mL after dilution. These results were in agreement with measurements using commercial immunoanalysis.

Recent advances in the detection of 17β-estradiol in food matrices: A review

Pollution of endocrine disrupting chemicals has become a global issue. As one of the hormonally active compounds, 17β-estradiol produces the strongest estrogenic effect when it enters the organism exogenously including food intakes, bringing potential harmfulness such as malfunction of the endocrine system. Therefore, in order to assure food safety and avoid potential risks of 17β-estradiol to humans, it is of great significance to develop rapid, sensitive and selective approaches for the detection of 17β-estradiol in food matrices. In this review, the harmfulness and main sources of 17β-estradiol are firstly introduced, followed by the description of the principles and applications of different approaches for 17β-estradiol detection including high performance liquid chromatography, electrochemistry, Raman spectroscopy, fluorescence and colorimetry. Particularly, applications in detecting 17β-estradiol in food matrices over the years of 2010-2018 are discussed. Finally, advantages and limitations of these detection methods are highlighted and perspectives on future developments in the detection methods for 17β-estradiol are also proposed. Although many detection approaches can achieve trace or ultratrace detection of 17β-estradiol, further studies should be focused on the development of in-situ and real-time methods to monitor and evaluate 17β-estradiol for food safety.

Electrochemical Biosensors as Potential Diagnostic Devices for Autoimmune Diseases

An important class of biosensors is immunosensors, affinity biosensors that are based on the specific interaction between antibodies and antigens. They are classified in four classes based on the type of employed transducer: electrochemical, optical, microgravimetric, and thermometric and depending on the type of recognition elements, antibodies, aptamers, microRNAs and recently peptides are integrating parts. Those analytical devices are able to detect peptides, antibodies and proteins in various sample matrices, without many steps of sample pretreatment. Their high sensitivity, low cost and the easy integration in point of care devices assuring portability are attracting features that justify the increasing interest in their development. The use of nanomaterials, simultaneous multianalyte detection and integration on platforms to form point-of-care devices are promising tools that can be used in clinical analysis for early diagnosis and therapy monitoring in several pathologies. Taking into account the growing incidence of autoimmune disease and the importance of early diagnosis, electrochemical biosensors could represent a viable alternative to currently used diagnosis methods. Some relevant examples of electrochemical assays for autoimmune disease diagnosis developed in the last several years based on antigens, antibodies and peptides as receptors were gathered and will be discussed further.

Electrodeposition of catalytically active polyporphyrin films of metal complexes of amino-substituted tetraphenylporphyrins

The process of electrodeposition of polyporphyrin films based on amino-substituted tetraphenylporphyrins and their metal complexes (CuT([Formula: see text]-NH[Formula: see text]Ph)P, FeClT([Formula: see text]-NH[Formula: see text]Ph)P, MnClT([Formula: see text]-NH[Formula: see text]Ph)P) has been studied by the quartz crystal microbalance method. The surfaces of the films have been characterized by the scanning electron microscopy method and the number of electrons taking part in porphyrin electropolymerization has been determined. It has been established that the most intensive film growth is observed for the metal-free porphyrin ligand. The obtained polyporphyrin films have been analyzed for the catalytic ability in oxygen electroreduction reactions.

Electrochemical integrated paper-based immunosensor modified with multi-walled carbon nanotubes nanocomposites for point-of-care testing of 17β-estradiol

17β-estradiol (17β-E2) plays a critical role in regulating reproduction in human, there is therefore an urgent need to detect it sensitively and precisely in a cost-effective and easy method. In this paper, a label-free integrated microfluidic paper-based analytical device was developed for highly sensitive electrochemical detection of 17β-E2. The microfluidic channel of the paper-based sensor was fabricated with wax printing and the three electrodes, including working, counter and reference electrode were screen-printed. Multi-walled carbon nanotubes (MWCNTs)/ thionine (THI)/ gold nanoparticles (AuNPs) Nano composites were synthesized and coated on screen-printed working electrode (SPWE) for the immobilization of anti-E2. In this electro-chemical system of paper-based immunoassay, THI molecules serving as an electrochemical mediator while MWCNTs and AuNPs, due to their excellent electrical conductivities, could accelerate electron transfer for the signal amplification. Experimental results revealed that the immunoassay is able to detect 17β-E2 as low as 10 pg mL^-1, with a linear range from 0.01 to 100 ng mL^-1. This microfluidic paper-based immunosensor would provide a new platform for low cost, sensitive, specific, and point-of-care diagnosis of 17β-E2.

Electrochemical sensing using magnetic molecularly imprinted polymer particles previously captured by a magneto-sensor

The determination of 1-chloro-2,4-dinitrobenzene (CDNB) was used as a proof-of-concept to a simple analytical practical configuration applying magnetic molecularly imprinted particles (mag-MIPs). Mag-MIPs were captured from an emulsion by a home-made magneto-sensor (where a small magnet was entrapped by a graphite-epoxy composite) and then, this sensor, was transferred to the solution containing the analyte, where, after binding to the mag-MIPs, the analyte was directly analysed using differential pulse voltammetry (DPV) since the magneto-sensor acted as the working electrode. After optimization, a detection limit of 6.0 μmol L^-1 with a RSD of 2.7% was achieved along with suitable recoveries and selectivity. This methodology offers a different approach for electroanalytical methodologies using mag-MIPs.

Detection of 17 β-Estradiol in Environmental Samples and for Health Care Using a Single-Use, Cost-Effective Biosensor Based on Differential Pulse Voltammetry (DPV)

Environmental estrogen pollution and estrogen effects on the female reproductive system are well recognized scientifically. Among the estrogens, 17 β-estradiol is a priority in environmental estrogen pollution, and it is also a major contributor to estrogen which regulates the female reproductive system. 17 β-estradiol is carcinogenic and has a tumor promotion effect relating to breast cancer, lung cancer and others. It also affects psychological well-being such as depression, fatigue and others. Thus, a simple method of detecting 17 β-estradiol will be important for both environmental estrogen pollution and health care. This study demonstrates a single-use, cost-effective 17 β-estradiol biosensor system which can be used for both environmental and health care applications. The bio-recognition mechanism is based on the influence of the redox couple, K₃Fe(CN)₆/K₄Fe(CN)₆ by the interaction between 17 β-estradiol antigen and its α-receptor (ER-α; α-estrogen antibody). The transduction mechanism is an electrochemical analytical technique, differential pulse voltammetry (DPV). The levels of 17 β-estradiol antigen studied were between 2.25 pg/mL and 2250 pg/mL; Phosphate buffered saline (PBS), tap water from the Cleveland regional water district, and simulated urine were used as the test media covering the potential application areas for 17 β-estradiol detection. An interference study by testosterone, which has a similar chemical structure and molecular weight as those of 17 β-estradiol, was carried out, and this 17 β-estradiol biosensor showed excellent specificity without any interference by similar chemicals.