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The combination of electrogenerated chemiluminescence (ECL) and aptamer-gated indicator delivering (gAID) magnetic mesoporous silica nanoparticles embedded into glass fibre paper functionalised with poly(ethyleneglycol) and N-(3-triethoxysilylpropyl)diethanolamine allowed the development of a rapid test that detects penicillin directly in diluted milk down to 50 ± 9 ppt in <5 min. Covalent attachment of the aptamer ‘cap’ to the silica scaffold enabled pore closure through non-covalent electrostatic interactions with surface amino groups, while binding of penicillin led to a folding-up of the aptamer thus releasing the ECL reporter Ru(bpy)32+ previously loaded into the material and letting it be detected after lateral flow by a smartphone camera upon electrochemical excitation with a screen printed electrode inserted into a 3D-printed holder. The approach is simple, generic and presents advantages with respect to sensitivity, measurement uncertainty and robustness compared with conventional fluorescence or electrochemical detection, especially for point-of-need analyses of challenging matrices and analytes at ultra-trace levels.
Rapid testing methods for the use directly at a point-of-need are expected to unfold their true potential especially when offering adequate capabilities for the simultaneous measurement of multiple analytes of interest. However, the implementation of multiplexing features while retaining simplicity, performance and portability is one of the prominent challenges in the field. Keeping in mind these challenges, we decided to combine the use of stimuli-responsive materials for small-molecule sensing relying on chemical signal amplification and their incorporation on paper strips for lateral-flow assays in a straightforward manner. Considering the modularity, high sensitivity and selectivity of antibody-gated indicator delivery systems, a multiplexed assay for three small-molecule explosives TATP, TNT and PETN was developed, allowing to detect the analytes simultaneously with a single test strip at lower ppb concentrations in liquid phase in <5 min, using a fluorescence reader or a smartphone for readout.
Because of the versatility of the hybrid material and the modularity of the assay architecture, it is obvious that this generic approach should be easily transferable to food or environmental analysis, point-of-care diagnostics and other areas of application in which the rapid screening for multiple parameters from liquid samples without clean-up in a dedicated laboratory is in demand.
The urgent necessity to carry out reliable and relevant analytical measurements directly at a point-of-need is one of the current drivers for the development of miniaturised analytical systems, quick tests and wearables. Despite their simplicity, this type of tests must guarantee analytical relevance and reliability like laboratory-based analysis, e.g., in terms of sensitivity, selectivity, immunity against false positives and false negatives as well as robustness and repeatability. Keeping in mind the high sensitivity offered by gated indicator-releasing micro- and nanoparticles due to their inherent features of signal amplification, we performed several optimisations to develop a potential biosensor platform for use in rapid tests. Conceptually, these gated materials are closely related to drug delivery systems, consisting of high porous materials usually closed with macromolecular “caps” and loaded with indicator molecules that are released in presence of a target analyte. However, the key difference between the two types of functional materials is that many drug delivery systems should deliver their cargo over a longer period, often many hours, whereas the gated materials prepared for sensing should show fast release kinetics, on the order of <5 min.
With the aim to optimise and adapt gated materials for sensing purposes, we prepared in this work several antibody-gated materials for small-molecule sensing. The materials consisted of porous silica particles containing indicator molecules in the pores and certain hapten molecules grafted to the particle surface close to the pore openings. The pores were then capped with antibodies binding to these haptens, thus inhibiting the escape of the indicators from inside of the pores. In presence of the corresponding analyte, the antibody is displaced from the surface of the material, allowing the escape of the indicators. This allows the detection of the analyte indirectly through an inherent signal amplification. In this work, the insecticide permethrin, a type-I pyrethroid, was selected as target model, because type-I pyrethroids play an important role in airplane disinfection. A first in-depth study of the various chemical tuning options of such antibody gated systems was performed. Different mesoporous silica supports, different functionalisation routes and different loading sequences were assessed. The materials’ performances were evaluated by studying their temporal response behaviour and detection sensitivity, including the tightness of pore closure (through the amount of blank release in absence of analyte) and the release kinetics. Our results indicate that the better the paratope-accommodating Fab region of the antibody “cap” fits into the host material’s pore openings, the better the closing/opening mechanism can be controlled. Because such materials can be used in various different formats from suspension assays[1] via microfluidic chips[2] to test strip-based lateral flow assays,[3] such materials present a powerful analytical particle platform for the sensitive analytics and diagnostics outside of a laboratory, realising sensitivities down to the µg kg–1 range in less analysis times of less than 5 min as we have recently demonstrated.[4]
The urgent necessity to carry out reliable and relevant analytical measurements directly at a point-of-need is one of the current drivers for the development of miniaturised analytical systems, quick tests and wearables. Despite their simplicity, this type of tests must guarantee analytical relevance and reliability like laboratory-based analysis, e.g., in terms of sensitivity, selectivity, immunity against false positives and false negatives as well as robustness and repeatability. Keeping in mind the high sensitivity offered by gated indicator-releasing micro- and nanoparticles due to their inherent features of signal amplification, we performed several optimisations to develop a potential biosensor platform for use in rapid tests. Conceptually, these gated materials are closely related to drug delivery systems, consisting of high porous materials usually closed with macromolecular “caps” and loaded with indicator molecules that are released in presence of a target analyte. However, the key difference between the two types of functional materials is that many drug delivery systems should deliver their cargo over a longer period, often many hours, whereas the gated materials prepared for sensing should show fast release kinetics, on the order of <5 min.
With the aim to optimise and adapt gated materials for sensing purposes, we prepared in this work several antibody-gated materials for small-molecule sensing. The materials consisted of porous silica particles containing indicator molecules in the pores and certain hapten molecules grafted to the particle surface close to the pore openings. The pores were then capped with antibodies binding to these haptens, thus inhibiting the escape of the indicators from inside of the pores. In presence of the corresponding analyte, the antibody is displaced from the surface of the material, allowing the escape of the indicators. This allows the detection of the analyte indirectly through an inherent signal amplification. In this work, the insecticide permethrin, a type-I pyrethroid, was selected as target model, because type-I pyrethroids play an important role in airplane disinfection. A first in-depth study of the various chemical tuning options of such antibody gated systems was performed. Different mesoporous silica supports, different functionalisation routes and different loading sequences were assessed. The materials’ performances were evaluated by studying their temporal response behaviour and detection sensitivity, including the tightness of pore closure (through the amount of blank release in absence of analyte) and the release kinetics. Our results indicate that the better the paratope-accommodating Fab region of the antibody “cap” fits into the host material’s pore openings, the better the closing/opening mechanism can be controlled. Because such materials can be used in various different formats from suspension assays[1] via microfluidic chips[2] to test strip-based lateral flow assays,[3] such materials present a powerful analytical particle platform for the sensitive analytics and diagnostics outside of a laboratory, realising sensitivities down to the µg kg–1 range in less analysis times of less than 5 min as we have recently demonstrated.[4]
Multiplexed bead-based array formats play an increasingly important role in analytical laboratories. Due to the high surface-to-volume ratio, fast reaction kinetics and modular assay design, these sensor formats are applied in clinical diagnostic, drug development and classical biosensors with great success.
As the spherical platform, researchers utilize micron sized particles made from polymeric or silica material. Such beads are commercially available from vendors such as BD or Luminex. However, we have encountered several problematic issues which accompany these platforms: first, bigger sized beads, which are required for particle handling reasons, are difficult to prepare with high monodispersity - a key requirement for cytometric application. Second, plain beads, made from either polymer or silica, have each several disadvantages such as inferior scattering properties in case of silica or limited flexibility for coupling strategies in the case of latex beads.
In order to overcome this problem, we have developed a versatile core-shell (CS) platform which consists of a polymeric core with a structurally controlled silica shell. In our approach, the core building block can be easily prepared with high yields and high monodispersity in a dispersion polymerization from approximately 500 nm to 1.3 µm. Then, silica is coated in a classical sol-gel process to protect the core with a stable yet modifiable surface (see SEM image in Figure 1, platform). Here, we combine ideal scattering properties and easy preparation of the polymeric core with the chemical flexibility of a silica surface. Moreover, the additional shell domain adds density to the composite, which makes particle handling feasible also for nanometer sized beads.
In this contribution we present proof-of-principle results of competitive immunoassays with fluorescence detection using our CS beads – each performed in mix-and-read fashion without washing steps. All sizes are applicable in cytometric read-out which and can be used for size encoding (see Set 1 to 3 in Figure 1, size encoding). However, further multiplexing for a set of at least 20 parameters can be achieved by swelling hydrophobic dyes into the core. To the same time, precise tuning of the surface with mixed silane layers allowed us to improve the selectivity towards small molecules in competitive immunoassays significantly (see example in Figure 1, Application). We believe that our platform allows researchers to gain access to superior assay performance in combination with a low-threshold approach for the synthesis of the spherical platform.
Gated hybrid delivery systems: En route to sensory materials with inherent signal amplification
(2013)
Hybrid nanoparticles with a large specific surface area are a particularly exciting vehicle for delivery applications. Such highly porous or container-like structures, usually prepared from silica and frequently from gold, can incorporate a large number of chemical substances such as drug and/or indicator molecules. When equipped with a chemically or physically addressable gating function at the openings of the voids, the release of the cargo can be controlled at will. Because many more molecules can be stored as cargo in the pores of the support than there are functional groups as anchoring sites for the gating entities attached to the outer surface (for efficient pore capping), the systems possess inherent features of (signal) amplification. The present article will introduce various design strategies for different types of physical (light, temperature, magnetism) and chemical (pH, metal ions, anion, small organic molecules, enzymes) stimuli in connection with drug and indicator release. We will highlight exciting aspects of combining both features in theranostic applications and will stress which requirements still have to be met by many of the systems to be readily applicable in a sensory context.
Nanosilver is increasingly used in optics, medicine and analytical chemistry. We recently reported on the synthesis and properties of novel peptide-coated chiral nanosilver [1] using a small hexapeptide based on the amino acids CKK. In a continuation of our previous work, we use the peptides to catalyse TEOS hydrolysis in order to form a dense silica layer shell around a single nanoparticle, preventing chemical etching, allowing their inclusion in other inorganics, and making them biocompatible. Because of mild reaction conditions, the peptide integrity is ensured, as the chiral information which is contained in the nanoparticle. Moreover, these novel core-shell structures remain well-dispersed and are biocompatible. The possibility of further processing (creation of metamaterials etc.) is also in the focus of our interest.
Simple tripeptides are scaffolds for the synthesis and further assembly of peptide/silver nanoparticle composites. Herein, we further explore peptide-controlled silver nanoparticle assembly processes. Silver nanoparticles with a pH-responsive peptide coating have been synthesized by using a one-step precipitation/coating route. The nature of the peptide/silver interaction and the effect of the peptide on the formation of the silver particles have been studied via UV/Vis, X-ray photoelectron, and surface-enhanced Raman spectroscopies as well as through electron microscopy, small angle X-ray scattering and powder X-ray diffraction with Rietveld refinement. The particles reversibly form aggregates of different sizes in aqueous solution. The state of aggregation can be controlled by the solution pH value. At low pH values, individual particles are present. At neutral pH values, small clusters form and at high pH values, large precipitates are observed.
El desarrollo de receptores sintéticos complejos basados en conceptos de la química supramolecular no garantiza en ocasiones un reconocimiento molecular selectivo. Una alternativa en ciertos aspectos más simple y prometedora es el empleo de materiales híbridos orgánico-inorgánicos preparados mediante la funcionalización de sólidos inorgánicos porosos con receptores sintéticos adecuados. El anclaje de estos sistemas coordinantes a un soporte sólido da lugar a ciertos efectos sinérgicos que no están presentes ni en el material inicial ni en el receptor por separado y que sugieren que estos nuevos materiales híbridos pueden ser empleados en el desarrollo de nuevos sensores y nuevos protocolos de reconocimiento molecular y/o iónico.
The development of synthetic receptors based in supramolecular chemistry concepts will no allow a selective molecular recognition. A more simple, convenient and innovative approach relies on the use of organic-inorganic hybrid materials. These hybrid materials will be prepared by the grafting of certain synthetic receptors onto porous inorganic solids. The grafting of these coordinating systems onto a solid support leads to synergic effects that are hardly achievable with only the synthetic receptors or the solids alone. The presence of these synergic effects suggests that these organic-inorganic hybrid materials will be used for the development of novel sensory systems and novel molecular and/or ionic recognitionprotocols.