Biosensor based on hydrogel optical waveguide spectroscopy for the detection of 17β-estradiol

Nanostructured wearable electrochemical and biosensor towards healthcare management: a review

In recent years, there has been a rapid increase in demand for wearable sensors, particularly these tracking the surroundings, fitness, and health of people. Thus, selective detection in human body fluid is a demand for a smart lifestyle by quick monitoring of electrolytes, drugs, toxins, metabolites and biomolecules, proteins, and the immune system. In this review, these parameters along with the main features of the latest and mostly cited research work on nanostructured wearable electrochemical and biosensors are surveyed. This study aims to help researchers and engineers choose the most suitable selective and sensitive sensor. Wearable sensors have broad and effective sensing platforms, such as contact lenses, Google Glass, skin-patch, mouth gourds, smartwatches, underwear, wristbands, and others. For increasing sensor reliability, additional advancements in electrochemical and biosensor precision, stability in uncontrolled environments, and reproducible sample conveyance are necessary. In addition, the optimistic future of wearable electrochemical sensors in fields, such as remote and customized healthcare and well-being is discussed. Overall, wearable electrochemical and biosensing technologies hold great promise for improving personal healthcare and monitoring performance with the potential to have a significant impact on daily lives. These technologies enable real-time body sensing and the communication of comprehensive physiological information.

Hydrogel-Based Biosensors

There is an increasing interest in sensing applications for a variety of analytes in aqueous environments, as conventional methods do not work reliably under humid conditions or they require complex equipment with experienced operators. Hydrogel sensors are easy to fabricate, are incredibly sensitive, and have broad dynamic ranges. Experiments on their robustness, reliability, and reusability have indicated the possible long-term applications of these systems in a variety of fields, including disease diagnosis, detection of pharmaceuticals, and in environmental testing. It is possible to produce hydrogels, which, upon sensing a specific analyte, can adsorb it onto their 3D-structure and can therefore be used to remove them from a given environment. High specificity can be obtained by using molecularly imprinted polymers. Typical detection principles involve optical methods including fluorescence and chemiluminescence, and volume changes in colloidal photonic crystals, as well as electrochemical methods. Here, we explore the current research utilizing hydrogel-based sensors in three main areas: (1) biomedical applications, (2) for detecting and quantifying pharmaceuticals of interest, and (3) detecting and quantifying environmental contaminants in aqueous environments.

In Situ Monitoring of Rolling Circle Amplification on a Solid Support by Surface Plasmon Resonance and Optical Waveguide Spectroscopy

The growth of surface-attached single-stranded deoxyribonucleic acid (ssDNA) chains is monitored in situ using an evanescent wave optical biosensor that combines surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The "grafting-from" growth of ssDNA chains is facilitated by rolling circle amplification (RCA), and the gradual prolongation of ssDNA chains anchored to a gold sensor surface is optically tracked in time. At a sufficient density of the polymer chains, the ssDNA takes on a brush architecture with a thickness exceeding 10 μm, supporting a spectrum of guided optical waves traveling along the metallic sensor surface. The simultaneous probing of this interface with the confined optical field of surface plasmons and additional more delocalized dielectric optical waveguide modes enables accurate in situ measurement of the ssDNA brush thickness, polymer volume content, and density gradients. We report for the first time on the utilization of the SPR/OWS technique for the measurement of the RCA speed on a solid surface that can be compared to that in bulk solutions. In addition, the control of ssDNA brush properties by changing the grafting density and ionic strength and post-modification via affinity reaction with complementary short ssDNA staples is discussed. These observations may provide important leads for tailoring RCA toward sensitive and rapid assays in affinity-based biosensors.

Responsive Hydrogel Binding Matrix for Dual Signal Amplification in Fluorescence Affinity Biosensors and Peptide Microarrays

A combined approach to signal enhancement in fluorescence affinity biosensors and assays is reported. It is based on the compaction of specifically captured target molecules at the sensor surface followed by optical probing with a tightly confined surface plasmon (SP) field. This concept is utilized by using a thermoresponsive hydrogel (HG) binding matrix that is prepared from a terpolymer derived from poly(N-isopropylacrylamide) (pNIPAAm) and attached to a metallic sensor surface. Epi-illumination fluorescence and SP-enhanced total internal reflection fluorescence readouts of affinity binding events are performed to spatially interrogate the fluorescent signal in the direction parallel and perpendicular to the sensor surface. The pNIPAAm-based HG binding matrix is arranged in arrays of sensing spots and employed for the specific detection of human IgG antibodies against the Epstein-Barr virus (EBV). The detection is performed in diluted human plasma or with isolated human IgG by using a set of peptide ligands mapping the epitope of the EBV nuclear antigen. Alkyne-terminated peptides were covalently coupled to the pNIPAAm-based HG carrying azide moieties. Importantly, using such low-molecular-weight ligands allowed preserving the thermoresponsive properties of the pNIPAAm-based architecture, which was not possible for amine coupling of regular antibodies that have a higher molecular weight.

Simultaneous detection of diethylstilbestrol and estradiol residues with a single immunochromatographic assay strip

An immunochromatographic assay (ICA) based on competitive format was developed and validated for simultaneously rapid and sensitive detection of diethylstilbestrol (DES) and estradiol (E2) in milk and tissue samples. For this purpose, two monoclonal antibodies raised against those two estrogens were conjugated to gold nanoparticles and were applied to the conjugate pads of the test strip. The competitors of the DES-BSA/E2-BSA conjugates were immobilized onto a nitrocellulose membrane at two detection zones to form T1 and T2, respectively. The immunochromatographic assay had a visual detection limit of DES at 30 ng/g in milk powder, 25 ng/g in liquid milk, and 25 ng/g in shrimp tissue, respectively, and the results can be judged within 7-10 min. The visual detection limit of E2 was 75 ng/g in milk powder, 65 ng/g in liquid milk, and 60 ng/g in shrimp tissue, respectively, and the results can be judged within 3-4 min. It had advantages in easy operation without requiring sophisticated equipment and specialized skills. By testing thirty milk and shrimp tissue samples from the local market, the method was compared with the HPLC-MS / MS method, and there was no statistical difference between the two methods. Furthermore, the immunochromatographic assay had good specificity, simple procedure, and low cost. This protocol was well suited for the food safety monitoring and early warning.

Optimization Synthesis and Biosensing Performance of an Acrylate-Based Hydrogel as an Optical Waveguiding Sensing Film

The metal-clad leaky waveguide (MCLW) is an optical biosensor consisting of a metal layer and a low index waveguide layer on a glass substrate. This label-free sensor measures refractive index (RI) changes within the waveguide layer. This work shows the development and optimization of acrylate based-hydrogel as the waveguide layer formed from PEG diacrylate (PEGDA, M_n 700), PEG methyl ether acrylate (PEGMEA, M_n 480), and acrylate-PEG₂₀₀₀-NHS fabricated on a substrate coated with 9.5 nm of titanium. The acrylate-based hydrogel is a synthetic polymer, so properties such as optical transparency, porosity, and hydrogel functionalization by a well-controlled reactive group can be tailored for immobilization of the bioreceptor within the hydrogel matrix. The waveguide sensor demonstrated an equal response to solutions of identical RI containing small (glycerol) and large (bovine serum albumin; BSA) analyte molecules, indicating that the hydrogel waveguide film is highly porous to both sizes of molecule, thus potentially allowing penetration of a range of analytes within the porous matrix. The final optimized MCLW chip was formed from a total hydrogel concentration of 40% v/v of PEGMEA-PEGDA (M_n 700), functionalized with 2.5% v/v of acrylate-PEG₂₀₀₀-NHS. The sensor generated a single-moded waveguide signal with a RI sensitivity of 128.61 ± 0.15° RIU^-1 and limit of detection obtained at 2.2 × 10^-6 RIU with excellent signal-to-noise ratio for the glycerol detection. The sensor demonstrated RI detection by monitoring changes in the out-coupled angle resulting from successful binding of d-biotin to streptavidin immobilized on functionalized acrylate hydrogel, generating a binding signal of (12.379 ± 0.452) × 10^-3°.

A Self-Referenced Diffraction-Based Optical Leaky Waveguide Biosensor Using Photofunctionalised Hydrogels

We report a novel self-referenced diffraction-based leaky waveguide (LW) comprising a thin (~2 µm) film of a photofunctionalisable hydrogel created by covalent attachment of a biotinylated photocleavable linker to chitosan. Streptavidin attached to the chitosan via the photocleavable linker was selectively removed by shining 365 nm light through a photomask to create an array of strips with high and low loading of the protein, which served as sensor and reference regions respectively. The differential measurements between sensor and reference regions were used for measuring analytes (i.e., biotin protein A and IgG) while reducing environmental and non-specific effects. These include changes in temperature and sample composition caused by non-adsorbing and adsorbing species, leading to reduction in effects by ~98%, ~99%, and ~97% respectively compared to the absolute measurements. The novelty of this work lies in combining photofunctionalisable hydrogels with diffraction-based LWs for referencing. This is needed to realise the full potential of label-free optical biosensors to measure analyte concentrations in real samples that are complex mixtures, and to allow for sample analysis outside of laboratories where drifts and fluctuations in temperature are observed.

Smart Hydrogel Micromechanical Resonators with Ultrasound Readout for Biomedical Sensing

One of the main challenges for implantable biomedical sensing schemes is obtaining a reliable signal while maintaining biocompatibility. In this work, we demonstrate that a combination of medical ultrasound imaging and smart hydrogel micromechanical resonators can be employed for continuous monitoring of analyte concentrations. The sensing principle is based on the shift of the mechanical resonance frequencies of smart hydrogel structures induced by their volume-phase transition in response to changing analyte levels. This shift can then be measured as a contrast change in the ultrasound images due to resonance absorption of ultrasound waves. This concept eliminates the need for implanting complex electronics or employing transcutaneous connections for sensing biomedical analytes in vivo. Here, we present proof-of-principle experiments that monitor in vitro changes in ionic strength and glucose concentrations to demonstrate the capabilities and potential of this versatile sensing platform technology.

Fabrication of a mesoporous silica film based optical waveguide sensor for detection of small molecules

In this paper, a thick mesoporous silica film (MSF) (more than 700 nm) was fabricated via the two-step enhancing Stöber solution growth approach (ESSGA). According to the optimization based on the transfer matrix method, a thicker MSF sensor has higher waveguide index sensitivity and is more suitable for the adsorbent detection, while a thinner MSF sensor has higher covered medium index sensitivity and is more appropriate for non-adsorbent detection. The covered medium index sensitivity and refractive index resolution of the fabricated MSF optical waveguide sensor were calculated to be 53.18 deg/RIU and ${1.28}\; \times \;{{10}^{ - 6}}\;{\rm RIU}$1.28×10^-6RIU, respectively. For the detection of a small molecule, hexadecyltrimethylammonium bromide was used as a model of a small molecule to verify its sensing property and its limit of detection (LOD) as low as 1.879 nM was obtained. In order to detect heavy metal ions, the MSF was modified with an amino group by the post-grafted method. The response of the resonance angle shift is more sensitive to ${{\rm Pb}^{2 + }}$Pb²⁺ ion than ${{\rm Cu}^{2 + }}$Cu²⁺ ion and both their LODs could reach the nanomolar detection level; those are 17.30 and 6.44 nM, respectively.

Processing and modification of hydrogel and its application in emerging contaminant adsorption and in catalyst immobilization: a review

Due to the wonderful property of hydrogels, they can provide a platform for a wide range of applications. Recently, there is a growing research interest in the development of potential hydrogel adsorbents in wastewater treatment due to their adsorption ability toward aqueous pollutants. It is important to prepare such a hydrogel that possesses appropriate robustness, adsorption capacity, and adsorption efficiency to meet the need of water treatment. In order to improve the property of hydrogels, much effort has been made by researchers to modify hydrogels, among which incorporating inorganic components into the polymeric networks is the most common method, which can reduce the product cost and simplify the preparation procedure. Not only can hydrogel be applied as adsorbent, but it also can be used as matrix for catalyst immobilization. In this review, the key advancement on the preparation and modification of hydrogels is discussed, with special emphasis on the introduction of inorganic materials into polymeric networks and consequential changes in the properties of mechanical strength, swelling, and adsorption. Besides, hydrogels used as adsorbents for removal of dyes and inorganic pollutants have been widely explored, but their use for adsorbing emerging contaminants from aqueous solution has not received much attention. Thus, this review is mainly focused on hydrogels' application in removing emerging contaminants by adsorption. Furthermore, hydrogels can be also applied in immobilizing catalysts, such as enzyme and photocatalyst, to remove pollutants completely and avoid secondary pollution, so their progress as catalyst matrix is overviewed.

A feasibility study of a leaky waveguide aptasensor for thrombin

This proof-of-principle study demonstrates the feasibility of a leaky waveguide (LW) aptasensor, where aptamers were immobilised in a mesoporous chitosan waveguiding film for the detection of thrombin. This work has demonstrated that aptamers immobilised in hydrogels retain their affinity and selectivity towards their target and thus can be used as bioreceptors. The use of antibodies as bioreceptors for sensing thrombin is not viable because it is a serine protease, which will cleave the antibodies. Currently used assays based on clotting time and chromogenic/fluorogenic substrates have limited potential for thrombin measurement in whole blood. Using the initial binding rate over the first 5 min, the limit of detection of our LW aptasensor for thrombin was ∼22 nM. The sensor was tested with spiked serum samples, giving a reading of 46.1 ± 4.6 nM for a sample containing 50 nM thrombin. Our proposed sensor combines the robustness and low cost of aptamers as molecular recognition elements with the simple fabrication process of the chitosan-based leaky waveguide, making LW aptasensors highly attractive for applications in point-of-care diagnostics and healthcare monitoring.

Folding Paper-Based Aptasensor Platform Coated with Novel Nanoassemblies for Instant and Highly Sensitive Detection of 17β-Estradiol

Owing to its critical role in the development of female reproductive tissues and as a clinical biomarker, there is an urgent need to develop a rapid and cost-effective method to sensitively detect 17β-estradiol (E2). In this work, a folding aptasensor platform with microfluidic channels for the label-free electrochemical detection of E2 is described. The platform, designed with a delicate folding structure, integrating filter holes, microfluidic channels, reaction chambers, and a three-electrode system, is extremely easy to use. To increase the detection sensitivity and immobilize the aptamer, we synthesized a novel nanoassembly consisting of amine-functionalized single-walled carbon nanotube/new methylene blue/gold nanoparticles (AuNPs) and modified the working electrode with this nanoassembly. The calibration curve obtained from the experimental results exhibited a linear range between 10 pg mL^-1 and 500 ng mL^-1 (R² = 0.993), and a detection limit of 5 pg mL^-1 was achieved (S/N = 3). Furthermore, experiments to detect E2 in clinical serum were conducted, and the results were highly similar to those obtained using a large electrochemical luminescence apparatus. By integrating multiple functional components, adopting novel nanoassemblies, and using a folding structure, this paper-based platform not only has great potential as a simple and convenient integrated device for point-of-care testing of E2, but also as a portable, low-cost, and highly sensitive aptasensor platform capable of detecting many diagnostic biomarkers with the appropriate aptamers.

Evaluation of Molecularly Imprinted Polymers for Point-of-Care Testing for Cardiovascular Disease

Molecular imprinting is a rapidly growing area of interest involving the synthesis of artificial recognition elements that enable the separation of analyte from a sample matrix and its determination. Traditionally, this approach can be successfully applied to small analyte (<1.5 kDa) separation/ extraction, but, more recently it is finding utility in biomimetic sensors. These sensors consist of a recognition element and a transducer similar to their biosensor counterparts, however, the fundamental distinction is that biomimetic sensors employ an artificial recognition element. Molecularly imprinted polymers (MIPs) employed as the recognition elements in biomimetic sensors contain binding sites complementary in shape and functionality to their target analyte. Despite the growing interest in molecularly imprinting techniques, the commercial adoption of this technology is yet to be widely realised for blood sample analysis. This review aims to assess the applicability of this technology for the point-of-care testing (POCT) of cardiovascular disease-related biomarkers. More specifically, molecular imprinting is critically evaluated with respect to the detection of cardiac biomarkers indicative of acute coronary syndrome (ACS), such as the cardiac troponins (cTns). The challenges associated with the synthesis of MIPs for protein detection are outlined, in addition to enhancement techniques that ultimately improve the analytical performance of biomimetic sensors. The mechanism of detection employed to convert the analyte concentration into a measurable signal in biomimetic sensors will be discussed. Furthermore, the analytical performance of these sensors will be compared with biosensors and their potential implementation within clinical settings will be considered. In addition, the most suitable application of these sensors for cardiovascular assessment will be presented.

Method for Determining Average Iron Content of Ferritin by Measuring its Optical Dispersion

We report a method where the refractive index increments of an iron storage protein, ferritin, and apoferritin (ferritin minus iron) were measured over the wavelength range of 450-678 nm to determine the average iron content of the protein. The protein used in this study had ∼3375 iron atoms per molecule. The measurement of optical dispersion over the broad wavelength range was enabled by the use of mesoporous leaky waveguides (LWs) made of chitosan. We present a facile approach for fabricating mesoporous chitosan waveguides for improving the measurement sensitivity of macromolecules such as ferritin. Mesoporous materials allow macromolecules to diffuse into the waveguide, maximizing their interaction with the optical mode and thus increasing sensitivity by a factor of ∼9 in comparison to nonporous waveguides. The sensitivity was further improved and selectivity toward ferritin was achieved by the incorporation of antibodies in the waveguide. The method presented in this work is a significant advance over the state of the art method, the enzyme linked immunosorbent assay (ELISA) used in clinics, because it allows determining the average content of ferritin in a single step. The average iron content of ferritin is an important marker for conditions such as injury, inflammation, and infection. Thus, the approach presented here of measuring optical dispersion to determine the average iron content of ferritin has a significant potential to improve the point of care analysis of the protein for disease diagnosis and screening.

A novel thermometer-type hydrogel senor for glutathione detection

A thermometer-type visual sensor for glutathione (GSH) sensing was developed with stimulus-responsive fluorescent hydrogel which was obtained by using 5, 6-bicarboxylic fluorescein crossli`nked partly ammoniated polyacrylamide. Various experimental parameters such as the particle size of hydrogel, buffer solution and swelling time were optimized. It is accessible to measure the volume change of hydrogel with the sensor by reading the graduation on a pipette like thermometer with naked eye. The concentration of the GSH depended on the volume in a certain range as the signal. Satisfactory agreements between the sensor and HPLC results for atuomolan tablet assays indicated the capability of the thermometer-type sensors for the analysis of real samples. These findings proved the utility of stimulus-responsive, intelligent hydrogel and the suitability of thermometer-style visual sensor design for quantitative assays.

Light-Guiding Biomaterials for Biomedical Applications

Optical techniques used in medical diagnosis, surgery, and therapy require efficient and flexible delivery of light from light sources to target tissues. While this need is currently fulfilled by glass and plastic optical fibers, recent emergence of biointegrated approaches, such as optogenetics and implanted devices, call for novel waveguides with certain biophysical and biocompatible properties and desirable shapes beyond what the conventional optical fibers can offer. To this end, exploratory efforts have begun to harness various transparent biomaterials to develop waveguides that can serve existing applications better and enable new applications in future photomedicine. Here, we review the recent progress in this new area of research for developing biomaterial-based optical waveguides. We begin with a survey of biological light-guiding structures found in plants and animals, a source of inspiration for biomaterial photonics engineering. We describe natural and synthetic polymers and hydrogels that offer appropriate optical properties, biocompatibility, biodegradability, and mechanical flexibility have been exploited for light-guiding applications. Finally, we briefly discuss perspectives on biomedical applications that may benefit from the unique properties and functionalities of light-guiding biomaterials.

Asymmetric split H-shape nanoantennas for molecular sensing

In this paper we report on a very sensitive biosensor based on gold asymmetric nanoantennas that are capable of enhancing the molecular resonances of C-H bonds. The nanoantennas are arranged as arrays of asymmetric-split H-shape (ASH) structures, tuned to produce plasmonic resonances with reflectance double peaks within the mid-infrared vibrational resonances of C-H bonds for the assay of deposited films of the molecule 17β-estradiol (E2), used as an analyte. Measurements and numerical simulations of the reflectance spectra have enabled an estimated enhancement factor on the order of 10⁵ to be obtained for a thin film of E2 on the ASH array. A high sensitivity value of 2335 nm/RIU was achieved, together with a figure of merit of approximately 8. Our experimental results were corroborated using numerical simulations for the C-H stretch vibrational resonances from the analyte, superimposed on the plasmonic resonances of the ASH nanoantennas.

Self-tuning interfacial architecture for Estradiol detection by surface plasmon resonance biosensor

This study reports the operation principles for reusable SPR biosensors utilizing nanoscale-specific electrostatic levitation phenomena in their sensitive layer design. Functional macromolecular building blocks localized near the "charged" surface by a variety of weak electrostatic interactions create a flexible and structurally variable architecture. A proof-of-concept is demonstrated by an immunospecific detection of 17β-Estradiol (E2) following the competitive inhibition format. The sensing interfacial architecture is based on the BSA-E2 conjugate within the BSA matrix immobilized on the "charged" (as a result of guanidine thiocyanate treatment) gold surface at pH 5.0. Kinetic analysis for different E2 concentrations shows that using parameter β of the stretched exponential function ~(1-exp(-(t/τ)^β) as an analyte-specific response measure allows one to substantially decrease the low detection limit (down to 10^-3ng/ml) and increase the dynamic range (10^-3-10³ng/ml) of the SPR biosensor. Finally, it's concluded that the created interfacial architecture is a typical complex system, where SPR response is formed by the stochastic interactions within the whole variety of processes in the system. The E2 addition destroys the uniformity of the reaction space (where an interaction of the antibody (Ab) and the analog of E2 in the self-tuneable matrix takes place) by the redistribution of the immunospecific complexes Ab(E2)_x (x=0, 1, 2) dependent on E2 concentration. Binding dynamics changes are reflected in the values of β which summarize in compact form all "hidden" information specific for the evolving distributed interfacial system.

Size-controllable ultrathin carboxylated polypyrrole nanotube transducer for extremely sensitive 17β-estradiol FET-type biosensors

Size-controllable aptamer conjugated ultrathin carboxylated polypyrrole nanotubes (A-UCPPyNTs) were successfully fabricated as transducers in 17β-estradiol field-effect transistor (FET)-type biosensors which has extremely high sensitivity (∼1 fM) and unique selectivity.

Detection of 17β-estradiol in water samples by a novel double-layer molecularly imprinted film-based biosensor

This study reports a novel double-layer molecularly imprinted film (MIF)-based biosensor for rapid, sensitive and highly selective detection of small molecule 17β-estradiol (E2) that is frequently detected in environmental water samples. In this system, the modification of gold surface of SPR chip was performed by 1-dodecanethiol. Then double-layer MIF was generated on the 1-dodecanethiol modified gold surface. The non-modified and imprinted surfaces were characterized by atomic force microscopy (AFM), scanning electron microscope (SEM) and contact angle measurements. Analysis of surface plasmon resonance (SPR) spectroscopy showed that the imprinted sensing film displayed good selectivity for E2 compared to other analog molecules and NIF. A good linear relationship was obtained between the SPR angle and E2 concentrations over a range of 2.50×10(-13)-2.50×10(-)(9)mol/L (R(2)=0.993) with the lowest measurable concentration of 2.50×10(-13)mol/L. The sensor can be regenerated with the mixture of acetic acid and PBS buffer (v/v=1:9) as a desorption agent over tens of times without significant deterioration of the sensor performance. Potential interference of real environmental sample matrix was assessed by spiked samples in several waste seawater effluents. This portable sensor system can be successfully applied for on-site real-time inexpensive and easy-to-use monitoring of E2 or other small molecule pollutants in environmental samples such as effluents or water bodies.

A simple and label-free aptasensor based on nickel hexacyanoferrate nanoparticles as signal probe for highly sensitive detection of 17β-estradiol

A simple and label-free electrochemical aptasensor was developed for detecting 17β-estradiol (E2). To translate the binding events between aptamer and E2 into the measurable electrochemical signal, the nickel hexacyanoferrate nanoparticles (NiHCF NPs) as signal probe was in situ introduced on the electrode by a simple two-step deposition method, exhibiting well-defined peaks with good stability and reproducibility. Subsequently, Au nanoparticles (Au NPs) was covered on the NiHCF NPs, which not only provided a platform for immobilizing the aptamer by S-Au interaction, but further enhanced the conductivity and stability of the signal probe. With the addition of E2, the formation of E2-aptamer complexes on the sensing interface retarded the interfacial electron transfer reaction of the probe, resulting in the decrease of the electrochemical signal. E2 could be readily examined by measuring the signal change. A linear range of 1×10(-12)-6×10(-10) M was obtained with a low detection limit of 0.8×10(-12) M. The aptasensor also exhibited high specificity to E2 in control experiments employing seven endocrine disrupting compounds as the interferents that had similar structure or coexisted with E2 in the environment. Besides, the applicability of the aptasensor was successfully evaluated by determining E2 in the real samples.

Fabrication and optical sensing properties of mesoporous silica nanorod arrays

A mesoporous silica nanorod (MSNR) array on a gold (Au) film was fabricated and used as an optical waveguide (OWG) sensor. A resolution of the refractive index (RI) as high as 3.6 × 10⁻⁸ RIU was achieved.

Surface plasmon resonance sensor for femtomolar detection of testosterone with water-compatible macroporous molecularly imprinted film

A novel water-compatible macroporous molecularly imprinted film (MIF) has been developed for rapid, sensitive, and label-free detection of small molecule testosterone in urine. The MIF was synthesized by photo copolymerization of monomers (methacrylic acid [MAA] and 2-hydroxyethyl methacrylate [HEMA]), cross-linker (ethylene glycol dimethacrylate, EGDMA), and polystyrene nanoparticles (PS NPs) in combination with template testosterone molecules. The PS NPs and template molecules were subsequently removed to form an MIF with macroporous structures and the specific recognition sites of testosterone. Incubation of artificial urine and human urine on the MIF and the non-imprinted film (NIF), respectively, indicated undetectable nonspecific adsorption. Accordingly, the MIF was applied on a surface plasmon resonance (SPR) sensor for the detection of testosterone in phosphate-buffered saline (PBS) and artificial urine with a limit of detection (LOD) down to 10(-15)g/ml. To the best of our knowledge, the LOD is considered as one of the lowest among the SPR sensors for the detection of small molecules. The control experiments performed with analogue molecules such as progesterone and estradiol demonstrated the good selectivity of this MIF for sensing testosterone. Furthermore, this MIF-based SPR sensor shows high stability and reproducibility over 8months of storage at room temperature, which is more robust than protein-based biosensors.

Bloch surface wave-enhanced fluorescence biosensor

A new approach to signal amplification in fluorescence-based assays for sensitive detection of molecular analytes is reported. It relies on a sensor chip carrying a one-dimensional photonic crystal (1DPC) composed of two piled up segments which are designed to increase simultaneously the excitation rate and the collection efficiency of fluorescence light. The top segment supports Bloch surface waves (BSWs) at the excitation wavelength and the bottom segment serves as a Bragg mirror for the emission wavelength of used fluorophore labels. The enhancement of the excitation rate on the sensor surface is achieved through the resonant coupling to BSWs that is associated with strong increase of the field intensity. The increasing of collection efficiency of fluorescence light emitted from the sensor surface is pursued by using the Bragg mirror that minimizes its leakage into a substrate and provides its beaming toward a detector. In order to exploit the whole evanescent field of BSW, extended three-dimensional hydrogel-based binding matrix that is functionalized with catcher molecules is attached to 1DPC for capturing of target analyte from a sample. Simulations supported by experiments are presented to illustrate the design and determined the performance characteristics of BSW-enhanced fluorescence spectroscopy. A model immunoassay experiment demonstrates that the reported approach enables increasing signal to noise ratio, resulting in about one order of magnitude improved limit of detection (LOD) with respect to regular total internal reflection fluorescence (TIRF) configuration.