Holographic molecularly imprinted polymers for label-free chemical sensing

Holographic surface relief diffraction gratings made of hydrogels for direct label-free biosensing of IgGs

This work describes the development of a label-free (LF) biosensing platform for the direct detection of targets based on diffractive structures fabricated with acrylamide-based hydrogels biofunctionalized with proteins and antibodies. Hydrogels containing Bovine Serum Albumin protein (BSA) with different crosslinking degrees were synthesized and characterized to find the optimal conditions for the suitable fabrication of surface relief gratings (SRGs). The bioavailability of BSA-functionalized hydrogels for the specific recognition of anti-BSA antibodies was verified by fluorescence detection. After the hydrogel-based SRG fabrication, diffraction efficiency measures at two different laser wavelengths were used for the direct LF detection of anti-BSA antibodies. The limit of detection in the sub mg L-1 range was read. Additionally, SRGs were prepared with hydrogels biofunctionalized with anti-rabbit antibodies for the direct detection of IgGs from rabbit serum, obtaining similar analytical performance without the necessity of labeling or applying amplification strategies.

High Refractive Index, Low Birefringence Holographic Materials via the Homopolymerization of 1,2-Dithiolanes

High refractive index, low birefringence photopolymers were created via the radical-mediated, ring opening homopolymerization of 1,2-dithiolane functionalized monomers and were subsequently evaluated as holographic recording media. This investigation systematically characterized the reaction kinetics, thermodynamics, and volume shrinkage of the 1,2-dithiolane homopolymerization as well as the optical transparency, refractive index, birefringence, and holographic performance of multifunctional 1,2-dithiolane functionalized monomers and their resultant polymers. Real-time kinetic and thermodynamic analyses of a monofunctional 1,2-dithiolane monomer, lipoic acid methyl ester (LipOMe), indicated rapid monomer conversion, exceeding 90% in 60 s, with an overall enthalpy of reaction of 18 ± 1 kJ/mol. The ring-opening polymerization resulted in low shrinkage (10.6 ± 0.3 cm3/mol dithiolane) and a significant bulk refractive index increase (0.030 ± 0.003). The resulting photopolymers exhibited high optical transparency, minimal haze, and negligible birefringence, suggesting the potential of 1,2-homopolymers as optical materials. To further explore the specific capabilities for use as high-performance holographic recording applications, several multifunctional monomers were synthesized with the ethanedithiol lipoic acid monomer (EDT-Lip2) selected for experimentation. Holographic diffraction gratings written using this monomer achieved a peak-to-mean refractive index modulation of 0.008 with minimal haze and birefringence.

Optical Fiber-Based Polymer Microcantilever for Chemical Sensing: A Through-Fiber Fabrication Scheme

Fiber optics offer an emerging platform for chemical and biological sensors when engineered with appropriate materials. However, the large aspect ratio makes the optical fiber a rather challenging substrate for standard microfabrication techniques. In this work, the cleaved end of an optical fiber is used as a fabrication platform for cantilever sensors based on functional polymers. The through-fiber fabrication process is triggered by photo-initiated free-radical polymerization and results in a high-aspect-ratio polymer beam in a single step. The dynamic mode application of these cantilevers is first demonstrated in air. These cantilevers are then tuned for sensing applications, including humidity and chemical sensing based on molecularly imprinted polymers.

Inclusion Complexes of Photosensitizers with Cyclodextrins for Enhancing the Fabrication of Volume Grating

A supramolecule photosensitizer (supra-photosensitizer) based on the host-guest complexation of cyclodextrins and a bis-chalcone dye (BDEA) was prepared. Methyl-β-cyclodextrin (Me-β-CD), with the highest inclusion rate, was confirmed as the best host among the four cyclodextrins. The host-guest properties of Me-β-CD and BDEA were characterized by FTIR, XRD, ¹H NMR, PLQY, SEM, Job's plot, Benesi-Hildebrand plot, and others. Compared to the conventional photosensitizers, the supra-photosensitizers showed higher fluorescence emission and longer fluorescence lifetime whether as a powder or distributed in a film. The improvement in fluorescence coincided with improvement in photoinitiation efficiency and was proven to enhance the generation of volume grating. It is expected that the supra-photosensitizer may open a new avenue for the design of high-performance photoinitiation systems.

A Magnetic Nanoparticle-Doped Photopolymer for Holographic Recording

Functionalised holograms are important for applications utilising smart diffractive optical elements for light redirection, shaping and in the development of sensors/indicators. This paper reports on holographic recording in novel magnetic nanocomposites and the observed temperature change in dry layers and liquid samples exposed to alternating magnetic field (AMF). The nanocomposite consists of N-isopropylacrylamide (NIPA)-based polymer doped with magnetic nanoparticles (MNPs), and local heating is achieved through magnetic induction. Here, volume transmission holographic gratings (VTHGs) are recorded with up to 24% diffraction efficiency (DE) in the dry layers of magnetic nanocomposites. The dry layers and liquid samples are then exposed to AMF. Efficient heating was observed in the liquid samples doped with Fe3O4 MNPs of 20 nm average size where the temperature increased from 27 °C to 64 °C after 300 s exposure to 111 mT AMF. The temperature increase in the dry layers doped with the same nanoparticles after exposure to 4.4 mT AMF was observed to be 6 °C. No temperature change was observed in the undoped layers. Additionally, we have successfully recorded Denisyuk holograms in the magnetic nanocomposite materials. The results reveal that the magnetic nanocomposite layers are suitable for recording holograms and need further optimisation in developing holographic indicators for mapping AMFs.

Molecular imprinting and cladding produces antibody mimics with significantly improved affinity and specificity

Molecularly imprinted polymers (MIPs), as important mimics of antibodies, are chemically synthesized by polymerization in the presence of a target compound. MIPs have found wide applications in important fileds. However, the current molecular imprinting technology suffers from a dilemma; there is often a compromise between the best affinity and the best specificity for MIPs prepared under optimized conditions. Herein, we proposed a new strategy called molecular imprinting and cladding (MIC) to solve this issue. The principle is straightforward; after molecular imprinting, a chemically inert cladding thinlayer is generated to precisely cover non-imprinted area. We further proposed a special MIC approach for controllably engineering protein binders. The prepared cladded MIPs (cMIPs) exhibited significantly improved affinity and specificity. The general applicability of the proposed strategy and method was verified by engineering of cMIPs for the recognition of a variety of different proteins. The feasibility of cMIPs for real applications was demonstrated by fluorescence imaging of cancer cells against normal cells and immunoassay of C-peptide in human urine. This study opened up a new avenue for controllably engineering protein-specific antibody mimics with excellent recognition properties, holding great prospective in important applications such as disease diagnosis and nanomedicine.

Hydrogel-based holographic sensors and biosensors: past, present, and future

Hydrogel-based holographic sensors consist of a holographic pattern in a responsive hydrogel that diffracts light at different wavelengths depending on the dimensions and refractive index changes in the material. The material composition of hydrogels can be designed to be specifically responsive to different stimuli, and thus the diffraction pattern can correlate with the amount of analyte. According to this general principle, different approaches have been implemented to achieve label-free optical sensors and biosensors, with advantages such as easy fabrication or naked-eye detection. A review on the different approaches, sensing materials, measurement principles, and detection setups, and future perspectives is offered.

Holographic Photopolymer Material with High Dynamic Range (Δn) via Thiol-Ene Click Chemistry

A high-performance holographic recording medium was developed based on a unique combination of photoinitiated thiol-ene click chemistry and functional, linear polymers used as binders. Allyl reactive sites were incorporated along the backbone of the linear polymer binder to enable facile film casting and to facilitate cross-linking by photopolymerization of the thiol-ene monomers that also serve as the writing monomers in this distinctive approach to holographic materials. The allyl content and the ratio of the linear polymer to the writing monomers were varied to maximize and control the refractive index contrast. A blade-coating-based film preparation method was developed to form films from the mixture of linear polymer and the thiol-ene monomers. This approach results in a holographic material with a peak to mean index contrast (Δn) that reaches 0.04. The refractive index contrast was stable for at least two weeks. Haze in holograms with a high writing monomer loading was significantly reduced when a higher allyl content was incorporated into the binder, resulting in the lowest haze around 0.2%. Finally, the media exhibit high resolution as demonstrated by the ability to record reflection holograms with 140 nm pitch and diffraction efficiency in excess of 90%.

RAFT coupling chemistry: a general approach for post-functionalizing molecularly imprinted polymers synthesized by radical polymerization

Herein we describe a straightforward protocol for the surface functionalization of free-radically synthesized imprinted nanoparticles via polymer grafting.

Controllably Prepared Aptamer-Molecularly Imprinted Polymer Hybrid for High-Specificity and High-Affinity Recognition of Target Proteins

Molecularly imprinted polymers (MIPs) and aptamers, as effective mimics of antibodies, can overcome only some drawbacks of antibodies. Since they have their own advantages and disadvantages, the combination of MIPs with aptamers could be an ideal solution to produce hybrid alternatives with improved properties and desirable features. Although quite a few attempts have been made in this direction, a facile and controllable approach for the preparation of aptamer-MIP hybrids still remains lacking. Herein, we present a new approach for facile and controllable preparation of aptamer-MIP hybrids for high-specificity and high-affinity recognition toward proteins. An aptamer that can bind the glycoprotein alkaline phosphatase (ALP) with relative weak affinity and specificity was used as a ligand, and controllable oriented surface imprinting was carried out with an in-water self-polymerization system of dopamine. A thin layer of polydopamine was formed to cover the template to an appropriate thickness. After removing the template from the polymer, an aptamer-MIP hybrid with apparently improved affinity and specificity toward ALP was obtained, giving cross-reactivity of 3.2-5.6% and a dissociation constant of 1.5 nM. With this aptamer-MIP hybrid, a plasmonic immunosandwich assay (PISA) was developed. Reliable detection of ALP in human serum by the PISA was demonstrated.

Electrochemical protein recognition based on macromolecular self-assembly of molecularly imprinted polymer: a new strategy to mimic antibody for label-free biosensing

A versatile strategy, based on the use of an amphiphilic copolymer as a macromonomer, was developed for the preparation of a fully synthetic MIP (molecularly imprinted polymer) sensor for protein recognition. A UV-crosslinkable copolymer poly(DMA-co-HEA-co-St) (UPDHS) was designed and synthesized to assemble with the template protein in aqueous solution, resulting in the fabrication of protein imprinted polymeric nanoparticles. Linear macromolecular chains were used to protect the structural integrity of the protein, through which a 3D structure was formed around the protein molecule to generate recognition cavities. Then the nanoparticles were immobilized on the cleaned surface of a transducer as an MIP sensing platform. The resultant MIP coating was then irradiated via ultraviolet light to ensure that the recognition cavities were stable after UV curing. After protein extraction, recognition cavities complementary to the protein molecule in shape, size and chemical functionality were formed in the platform, which could then selectively rebind to the template in a mixture of closely related compounds. The sensor exhibited satisfactory selectivity, a wide linear range from 10^-14 to 10^-9 mg mL^-1, and a comparatively lower detection limit for protein detection. This strategy offers a new and straightforward method for the synthesis of receptors for label-free and cost-efficient protein recognition. This is one of the most effective and versatile strategies for the preparation of high-performance protein recognition devices based on a fully synthetic MIP.

Specific recognition of proteins and peptides via controllable oriented surface imprinting of boronate affinity-anchored epitopes

Molecularly imprinted polymers (MIPs) are chemically synthesized materials mimicking the recognition of antibodies towards antigens. Epitope imprinting has been an effective strategy, making imprinting of proteins flexible to a great extent. However, so far there is apparently a lack of facile and versatile epitope imprinting approaches. Herein, we present a new method called controllable oriented surface imprinting of boronate affinity-anchored epitopes. In this method, a C-terminus nonapeptide epitope was glycated and anchored as a template onto a boronic acid-functionalized substrate, followed by controllable oriented surface imprinting via the polycondensation of multiple silylating reagents containing functionalities capable of interacting with the epitope. The developed imprinting approach allowed for precise control of the thickness of the imprinting layer through adjusting the imprinting time, generating excellent binding properties. This method was verified to be versatile and efficient. Thus, it could greatly facilitate the preparation of MIPs for specific recognition of proteins and peptides.

Glycan-Imprinted Magnetic Nanoparticle-Based SELEX for Efficient Screening of Glycoprotein-Binding Aptamers

Nucleic acid aptamers, as useful alternatives of antibodies, have found a large range of promising applications such as affinity separation and bioassays. The screening of aptamers is critical for their applications. Aptamers are often screened by an in vitro methodology called SELEX (systematic evolution of ligands by exponential enrichment). Although numerous SELEX methods have been established to facilitate the selection, new efficient selection methods are still much needed. Molecularly imprinted polymers, which are antibody alternatives at the material level and competitors of aptamers, have not been used as a platform for aptamer selection yet so far. In this study, a glycan-imprinted magnetic nanoparticles (MNPs)-based SELEX was developed to efficiently screen aptamers against glycoproteins. Glycan-imprinted MNPs were used as an affinity interface to bind target glycoprotein, and then the target glycoprotein-bound MNPs were used as an affinity substrate for aptamer selection. The glycan-imprinted MNPs were synthesized by a state-of-the-art imprinting approach called boronate affinity controllable oriented surface imprinting. The glycan-imprinted MNPs exhibited high affinity and specificity and therefore allowed preferential binding toward target glycoproteins while excluding unwanted species. Two representative glycoproteins, including RNase B and transferrin, were employed as target glycoproteins, and aptamers with high affinity and specificity toward the two target glycoproteins were screened out in 3 rounds. This method exhibited some merits, such as high affinity, fast speed, and avoiding negative screening. Therefore, the glycan-imprinted MNP-based SELEX approach holds great values for the efficient screening of high-performance aptamers.

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

Self-processing photopolymer materials for versatile design and fabrication of holographic sensors and interactive holograms

The aim of this paper is to discuss the benefits as well as the limitations of utilizing photopolymer materials in the design of holograms that are responsive to changes in their environment, such as changes in the concentration of a specific substance, temperature, and pressure. Three different case studies are presented, including both surface and volume phase holograms, in order to demonstrate the flexibility in the approach of utilizing holographic photopolymers for the design of sensors and interactive optical devices. First, a functionalized surface relief hologram is demonstrated to operate as an optical sensor for the detection of metal ions in water. The sensitivity and selectivity of the sensor are investigated. The second example demonstrates a volume transmission hologram recorded in a temperature-sensitive photopolymer and the memory effects of its exposure to elevated temperature. Finally, a pressure-sensitive reflection hologram that changes color under application of pressure is characterized, and its potential application in document authentication is described.

Micro-patterned molecularly imprinted polymer structures on functionalized diamond-coated substrates for testosterone detection

Molecularly imprinted polymers (MIPs) can selectively bind target molecules and can therefore be advantageously used as a low-cost and robust alternative to replace fragile and expensive natural receptors. Yet, one major challenge in using MIPs for sensor development is the lack of simple and cost-effective techniques that allow firm fixation as well as controllable and consistent receptor material distribution on the sensor substrate. In this work, a convenient method is presented wherein microfluidic systems in conjunction with in situ photo-polymerization on functionalized diamond substrates are used. This novel strategy is simple, efficient, low-cost and less time consuming. Moreover, the approach ensures a tunable and consistent MIP material amount and distribution between different sensor substrates and thus a controllable active sensing surface. The obtained patterned MIP structures are successfully tested as a selective sensor platform to detect physiological concentrations of the hormone disruptor testosterone in buffer, urine and saliva using electrochemical impedance spectroscopy. The highest added testosterone concentration (500 nM) in buffer resulted in an impedance signal of 10.03 ± 0.19% and the lowest concentration (0.5 nM) led to a measurable signal of 1.8 ± 0.15% for the MIPs. With a detection limit of 0.5 nM, the MIP signals exhibited good linearity between a 0.5 nM and 20 nM concentration range. Apart from the excellent and selective recognition offered by these MIP structures, they are also stable during and after the dynamic sensor measurements. Additionally, the MIPs can be easily regenerated by a simple washing procedure and are successfully tested for their reusability.

Soluble Molecularly Imprinted Polymer-Based Potentiometric Sensor for Determination of Bisphenol AF

Molecularly imprinted polymer (MIP)-based polymeric membrane potentiometric sensors have been successfully developed for determination of organic compounds in their ionic and neutral forms. However, most of the MIP receptors in potentiometric sensors developed so far are insoluble and cannot be well dissolved in the polymeric membranes. The heterogeneous molecular recognitions between the analytes and MIPs in the membranes are inefficient due to the less available binding sites of the MIPs. Herein we describe a novel polymeric membrane potentiometric sensor using a soluble MIP (s-MIP) as a receptor. The s-MIP is synthesized by the swelling of the traditional MIP at a high temperature. The obtained MIP can be dissolved in the plasticized polymeric membrane for homogeneous binding of the imprinted polymer to the target molecules. By using neutral bisphenol AF as a model, the proposed method exhibits an improved sensitivity compared to the conventional MIP-based sensor with a lower detection limit of 60 nM. Moreover, the present sensor exhibits an excellent selectivity over other phenols. We believe that s-MIPs can provide an appealing substitute for the traditional insoluble MIP receptors in the development of polymeric membrane-based electrochemical and optical sensors.

Theoretical modeling and design of photonic structures in zeolite nanocomposites for gas sensing. Part I: surface relief gratings

The suitability of holographic structures fabricated in zeolite nanoparticle-polymer composite materials for gas sensing applications has been investigated. Theoretical modeling of the sensor response (i.e., change in hologram readout due to a change in refractive index modulation or thickness as a result of gas adsorption) of different sensor designs was carried out using Raman-Nath theory and Kogelnik's coupled wave theory. The influence of a range of parameters on the sensor response of holographically recorded surface and volume photonic grating structures has been studied, namely the phase difference between the diffracted and probe beam introduced by the grating, grating geometry, thickness, spatial frequency, reconstruction wavelength, and zeolite nanoparticle refractive index. From this, the optimum fabrication conditions for both surface and volume holographic gas sensor designs have been identified. Here, in part I, results from theoretical modeling of the influence of design on the sensor response of holographically inscribed surface relief structures for gas sensing applications is reported.

Synthesis of molecularly imprinted polymers by photo-iniferter polymerization under visible light

A new prospect for the synthesis of molecularly imprinted polymers: photo-iniferter polymerization under visible light.

Methanol Gas-Sensing Properties of SWCNT-MIP Composites

The single-walled carbon nanotube (SWCNT)-molecularly imprinted powder (MIP) composites in this paper were prepared by mixing SWCNTs with MIPs. The structure and micrograph of the as-prepared SWCNTs-MIPs samples were characterized by XRD and TEM. The gas-sensing properties were tested through indirect-heating sensors based on SWCNT-MIP composites fabricating on an alumina tube with Au electrodes and Pt wires. The results showed that the structure of SWCNTs-MIPs is of orthogonal perovskite and the average particle size of the SWCNTs-MIPs was in the range of 10-30 nm. SWCNTs-MIPs exhibit good methanol gas-sensitive properties. At 90 °C, the response to 1 ppm methanol is 19.7, and the response to the interferent is lower than 5 to the other interferent gases (ethanol, formaldehyde, toluene, acetone, ammonia, and gasoline). The response time and recovery time are 50 and 58 s, respectively. The as prepared SWCNTs-MIPs possesses good selectivity and high response to low concentrationmethanol.

Rapid Prototyping of Chemical Microsensors Based on Molecularly Imprinted Polymers Synthesized by Two-Photon Stereolithography

Two-photon stereolithography is used for rapid prototyping of submicrometre molecularly imprinted polymer-based 3D structures. The structures are evaluated as chemical sensing elements and their specific recognition properties for target molecules are confirmed. The 3D design capability is exploited and highlighted through the fabrication of an all-organic molecularly imprinted polymeric microelectromechanical sensor.

Highly stable and reusable imprinted artificial antibody used for in situ detection and disinfection of pathogens

Sandwich ELISA methods have been widely used for biomarker and pathogen detection because of their high specificity and sensitivity. However, the main drawbacks of this assay are the cost, the time-consuming procedure for the isolation of antibodies and their poor stability. To overcome these restrictions, we herein fabricated artificial antibodies based on imprinting technology and developed a sandwich ELISA for pathogen detection. Both the capture and detection antibodies were obtained via an in situ method, with simplicity, rapidity and low cost. The peroxidase mimics, the CeO2 nanoparticles, as signal generators were integrated with the detection antibody. The fabricated artificial antibodies exhibited not only natural antibody-like binding affinities and selectivities, but also superior stability and reusability. The detection limit was about 500 CFU mL-1, which is much lower than that of traditional ELISA methods (104 to 105 CFU mL-1). Furthermore, the capture antibody can disinfect pathogens in situ.

Cell-imprinted antimicrobial bionanomaterials with tolerable toxic side effects

Taking advantage of imprinting technology, artificial antibody-microbial imprinted Ag-TiO(2) materials are fabricated for microbial inactivation using a facile and green method. Due to the induced shape and size recognition elements, the artificial antibodies specifically recognize and kill target microbes under visible-light irradiation with minimal toxic side effects toward mammalian cells.

Easily crosslinkable side-chain azobenzene polymers for fast and persistent fixation of surface relief gratings

A facile and highly efficient approach to achieve rapid and persistent fixation of the photoinduced surface relief gratings is described.

All-organic microelectromechanical systems integrating specific molecular recognition--a new generation of chemical sensors

Cantilever-type all-organic microelectromechanical systems based on molecularly imprinted polymers for specific analyte recognition are used as chemical sensors. They are produced by a simple spray-coating-shadow-masking process. Analyte binding to the cantilever generates a measurable change in its resonance frequency. This allows label-free detection by direct mass sensing of low-molecular-weight analytes at nanomolar concentrations.

Hydrogel-Based Molecularly Imprinted Polymers for Biological Detection

Molecularly imprinted polymers (MIPs) have become an important tool in the preparation of artificial and robust recognition materials that are capable of mimicking natural systems. MIPs have been regarded as 'antibody mimics' and have shown clear advantages over real antibodies for sensor technology. Currently, on-site diagnostic (OSD) and point-of-care (POC) biosensor development are heavily dominated by antibody-dependent immuno-sensors such as the lateral flow immuno-assay. Although antibodies exhibit a high degree of selectivity, any biological recognition element is inherently unstable with limited shelf-life, even when stored under optimum conditions. OSD and POC tests are essential for disease screening and treatment monitoring as part of emergency management. Introduced or naturally occurring pathogens can cause significant disruptions, raise panic in the population, and result in significant economic losses. Cheaper, smaller, and smarter devices for early detection of disease or environmental hazards ultimately lead to rapid containment and corrective action. To this end, there has been extensive research on detection platforms based on genetic or immune techniques. MIPs have proven to produce selective biological extractions that rival immunoaffinity-based separations, but without the tediously lengthy time-consuming process. MIPs could provide an alternative to antibodies, and ultimately lead to cheaper, smaller, and smarter biosensors.

Computational modelling and characterisation of nanoparticle-based tuneable photonic crystal sensors

Photonic crystals are materials that are used to control or manipulate the propagation of light through a medium for a desired application.