Polymer- and colloid-mediated bioassays, sensors and diagnostics

Poly(ethylene glycol) diacrylate based hydrogel filled micropore with enhanced sensing capability

Ionic current rectification (ICR) phenomena conventionally occurs in nanopores which dimensions are comparable to the thickness of their electrical double layers. However, the microscale ICR in a micropore can also exist under some conditions. Here, the charged hydrogel filled conical micropore was constructed to realize microscale ICR. To better understand the micropore ICR, the influences of space charge density of the hydrogel, micropore geometry, the hydrogel filling length as well as the electrolyte concentration and pH were investigated. Furthermore, we developed a PEGDA-based hydrogel filled micropore sensing platform which sensing performance was enhanced due to the weakly charged PEGDA. The promyelocytic leukemia (PML)/retinoic acid receptor alpha (RARA) fusion genes and adenosine triphosphate (ATP) were respectively used as model analytes and the measured detection limits of 0.1 pM were achieved. The successful realization of microscale ICR in a homogenous and functional hydrogel filled micropore suggests that the fabrication, characterization and operation of ICR based devices can be more robust and facilitated for the wider applications.

Patterning of protein-based materials

Micro- and nanopatterning of proteins on surfaces allows to develop for example high-throughput biosensors in biomedical diagnostics and in general advances the understanding of cell-material interactions in tissue engineering. Today, many techniques are available to generate protein pattern, ranging from technically simple ones, such as micro-contact printing, to highly tunable optical lithography or even technically sophisticated scanning probe lithography. Here, one focus is on the progress made in the development of protein-based materials as positive or negative photoresists allowing micro- to nanostructured scaffolds for biocompatible photonic, electronic and tissue engineering applications. The second one is on approaches, which allow a controlled spatiotemporal positioning of a single protein on surfaces, enabled by the recent developments in immobilization techniques coherent with the sensitive nature of proteins, defined protein orientation and maintenance of the protein activity at interfaces. The third one is on progress in photolithography-based methods, which allow to control the formation of protein-repellant/adhesive polymer brushes.

Nanoparticle Tracking to Probe Transport in Porous Media

From the granular and fractured subsurface environment to highly engineered polymer membranes used in pharmaceutical purification, porous materials are ubiquitous in nature and industrial applications. In particular, porous media are used extensively in processes including water treatment, pharmaceutical sterilization, food/beverage processing, and heterogeneous catalysis, where hindered mass transport is either essential to the process or a necessary but undesirable limitation. Unfortunately, there are currently no universal models capable of predicting mass transport based on a description of the porous material because real porous materials are complex and because many coupled dynamic mechanisms (e.g., adsorption, steric effects, hydrodynamic effects, electrostatic interactions, etc.) give rise to the observed macroscopic transport phenomena.While classical techniques, like nuclear magnetic resonance and dynamic light scattering, provide useful information about mass transport in porous media at the ensemble level, they provide limited insight into the microscopic mechanisms that give rise to complex phenomena such as anomalous diffusion, hindered pore-space accessibility, and unexpected retention under flow, among many others. To address this issue, we have developed refractive index matching imaging systems, combined with single-particle tracking methods, allowing the direct visualization of single-particle motion within a variety of porous materials.In this Account, we summarize our recent efforts to advance the understanding of nanoparticle transport in porous media using single-particle tracking methods in both fundamental and applied scenarios. First, we describe the basic principles for two-dimensional and three-dimensional single-particle tracking in porous materials. Then, we provide concrete examples of nanoparticle transport in porous materials from two perspectives: (1) understanding fundamental elementary particle transport processes in porous media, including pore accessibility and cavity escape, which limit transport in porous media, and (2) facilitating applications in industrial processes, e.g., by understanding the mechanisms of particle fouling and remobilization in filtration membranes. Finally, we provide an outlook of opportunities associated with investigating other types of mass transport in confined environments using single-particle tracking methods, including electrophoretic and self-propelled motion.

Responsive polymers for medical diagnostics

Stimulus-responsive polymers have been used in improving the efficacy of medical diagnostics through different approaches including enhancing the contrast in imaging techniques and promoting the molecular recognition in diagnostic assays. This review overviews the mechanisms of stimulus-responsive polymers in response to external stimuli including temperature, pH, ion, light, etc. The applications of responsive polymers in magnetic resonance imaging, capture and purification of biomolecules through protein-ligand recognition and lab-on-a-chip technology are discussed.

Functionalized DNA-spider silk nanohydrogels for controlled protein binding and release

Hydrogels are excellent scaffolds to accommodate sensitive enzymes in a protective environment. However, the lack of suitable immobilization techniques on substrates and the lack of selectivity to anchor a biocatalyst are major drawbacks preventing the use of hydrogels in bioanalytical devices. Here, nanofilm coatings on surfaces were made of a recombinant spider silk protein (rssp) to induce rssp self-assembly and thus the formation of fibril-based nanohydrogels. To functionalize spider silk nanohydrogels for bioselective binding of proteins, two different antithrombin aptamers were chemically conjugated with the rssp, thereby integrating the target-binding function into the nanohydrogel network. Human thrombin was selected as a sensitive model target, in which the structural integrity determines its activity. The chosen aptamers, which bind various exosites of thrombin, enabled selective and cooperative embedding of the protein into the nanohydrogels. The change of the aptamer secondary structure using complementary DNA sequences led to the release of active thrombin and confirmed the addressable functionalization of spider silk nanohydrogels.

Catalytic Janus Colloids: Controlling Trajectories of Chemical Microswimmers

Catalytic Janus colloids produce rapid motion in fluids by decomposing dissolved fuel. There is great potential to exploit these "autonomous chemical swimmers" in applications currently performed by diffusion limited passive colloids. Key application areas for colloids include transporting active ingredients for drug delivery, gathering analytes for medical diagnostics, and self-assembling into regular structures used for photonic materials and lithographic templating. For drug delivery and medical diagnostics, controlling colloidal motion is key in order to target therapies, and transport analytes through lab-on-a-chip devices. Here, the autonomous motion of catalytic Janus colloids can remove the current requirements to induce and control colloid motion using external fields, thereby reducing the technological complexity required for medical therapies and diagnostics. For materials applications exploiting colloidal self-assembly, the additional interactions introduced by catalytic activity and rapid motion are predicted to allow access to new reconfigurable and responsive structures. In order to realize these goals, it is vital to develop methods to control both individual colloidal paths and collective behavior in motile catalytic colloidal systems. However, catalytic Janus colloids' trajectories are randomized by Brownian effects, and so require new strategies in order to be harnessed for transport. This is achievable using a variety of different approaches. For example, self-assembly and control of catalyst geometry can introduce controlled amounts of rotary motion, or "spin" into chemical swimmer trajectories. Furthermore, rotary motion combined with gravity, produces well-defined orientated helical trajectories. In addition, when catalytic colloids interact with topographical features, such as edges and trenches, they are steered. This gives rise to a new approach for autonomous colloidal microfluidic transport that could be deployed in future lab-on-a-chip devices. Chemical gradients can also influence the motion of catalytic Janus colloids, for example, to cause collective accumulations at specific locations. However, at present, the predicted theoretical degree of control over this phenomenon has not been fully verified in experimental systems. Collective behavior control for chemical swimmers is also possible by exploiting the potential for the complex interactions in these systems to allow access to self-assembled, dynamic and reconfigurable ordered structures. Again, current experiments have not yet accessed the breadth of possible behavior. Consequently, continued efforts are required to understand and control these interaction mechanisms in real world systems. Ultimately, this will help realize the use of catalytic Janus colloids for tasks that require well-controlled motion and structural organization, enabling functions such as analyte capture and concentration, or targeted drug delivery.

Design of polymer particle dispersions (latexes) in the course of radical heterophase polymerization for biomedical applications

Dispersions of polymer particle (DPPs) are increasingly being exploited both as biomolecule carriers, and as markers in various DPP biomedical applications related to cell and molecular biology, enzymology, immunology, diagnostics, in vitro and in vivo visualization, bioseparation, etc. Their potential to reduce reaction scales, lower costs, improve the rate, sensitivity, selectivity, stability and reproducibility of assays governs the diversity of their bioapplications. This review focuses on the design of DPPs with innovative special properties in the course of free radical heterophase polymerization that provides careful control of both macromolecular and colloidal properties. We demonstrate approaches that, according to the polymerization technique, regulate the particle size, shape, particle size distribution, morphology, surface chemistry and functionality, as well as the formation of organic-inorganic hybrid DPPs. The production of bioreagents based on DPPs and their use in bioassay are also reviewed.

Hydrogel Based Biosensors for In Vitro Diagnostics of Biochemicals, Proteins, and Genes

Hydrogel-based biosensors have drawn considerable attention due to their various advantages over conventional detection systems. Recent studies have shown that hydrogel biosensors can be excellent alternative systems to detect a wide range of biomolecules, including small biochemicals, pathogenic proteins, and disease specific genes. Due to the excellent physical properties of hydrogels such as the high water content and stimuli-responsive behavior of cross-linked network structures, this system can offer substantial improvement for the design of novel detection systems for various diagnostic applications. The other main advantage of hydrogels is the role of biomimetic three-dimensional (3D) matrix immobilizing enzymes and aptamers within the detection systems, which enhances their stability. This provides ideal reaction conditions for enzymes and aptamers to interact with substrates within the aqueous environment of the hydrogel. In this review, we have highlighted various novel detection approaches utilizing the outstanding properties of the hydrogel. This review summarizes the recent progress of hydrogel-based biosensors and discusses their future perspectives and clinical limitations to overcome.

Improving bioassay sensitivity through immobilization of bio-probes onto reactive micelles

We report a straightforward approach based on reactive copolymer micelles to improve bioassay sensitivity through enhanced probe accessibility.

Engineered synthetic polymer nanoparticles as IgG affinity ligands

A process for the preparation of an abiotic protein affinity ligand is described. The affinity ligand, a synthetic polymer hydrogel nanoparticle (NP), is formulated with functional groups complementary to the surface presentation of the target protein. An iterative process is used to improve affinity by optimizing the composition and proportion of functional monomers. Since the polymer NPs are formed by a kinetically driven process, the sequence of functional monomers in the polymer chain is not controlled; only the average composition can be adjusted by the stoichiometry of the monomers in the feed. To compensate for this the hydrogel NP is lightly cross-linked resulting in chain flexibility that takes place on a submillisecond time scale allowing the polymer to "map" onto a protein surface with complementary functionality. In this study, we report a lightly cross-linked (2%) N-isopropyl acrylamide (NIPAm) synthetic polymer NP (50-65 nm) incorporating hydrophobic and carboxylate groups that binds with high affinity to the Fc fragment of IgG. The affinity and amount of NP bound to IgG is pH dependent. The hydrogel NP inhibits protein A binding to the Fc domain at pH 5.5, but not at pH 7.3. A computational analysis was used to identify potential NP-protein interaction sites. Candidates include a NP binding domain that overlaps with the protein A-Fc binding domain at pH 5.5. The computational analysis supports the inhibition experimental results and is attributed to the difference in the charged state of histidine residues. Affinity of the NP (3.5-8.5 nM) to the Fc domain at pH 5.5 is comparable to protein A at pH 7. These results establish that engineered synthetic polymer NPs can be formulated with an intrinsic affinity to a specific domain of a large biomacromolecule.

Optical scattering artifacts observed in the development of multiplexed surface enhanced Raman spectroscopy nanotag immunoassays

Here we describe scattering based signal suppression artifacts encountered while developing multiplex lateral flow (LF) immunoassay using surface enhanced Raman spectroscopy (SERS) "nanotags" as analyte labels. Using these SERS nanotags, we have produced a quantitative test for inflammation biomarkers that is transferable to the point of care (POC). The SERS assay shows similar performance when compared with a fluorescent nanoparticle POC test. Here, using cardiac and inflammation biomarkers, we highlight the need to carefully optimize the concentration of assay components when using SERS nanotags and a single-line multiplexing approach. We show that in certain circumstances the SERS signal may be suppressed, leading to a significant underestimation of the analyte concentrations. Using electron microscopy and optical spectroscopy, we demonstrate that the error in the measurement is associated with the light scattering properties of the nanotags. These findings will be applicable to other nanoparticle labels with high light scattering coefficients. Through careful modification of the assay to reduce the impact of light scattering, it is possible to produce quantitative assays, but potentially at the expense of multiplexing capability and assay sensitivity.

Critical issues in sensor science to aid food and water safety

The stability of food and water supplies is widely recognized as a global issue of fundamental importance. Sensor development for food and water safety by nonconventional assays continues to overcome technological challenges. The delicate balance between attaining adequate limits of detection, chemical fingerprinting of the target species, dealing with the complex food matrix, and operating in difficult environments are still the focus of current efforts. While the traditional pursuit of robust recognition methods remains important, emerging engineered nanomaterials and nanotechnology promise better sensor performance but also bring about new challenges. Both advanced receptor-based sensors and emerging non-receptor-based physical sensors are evaluated for their critical challenges toward out-of-laboratory applications.

Enhancing single molecule imaging in optofluidics and microfluidics

Microfluidics and optofluidics have revolutionized high-throughput analysis and chemical synthesis over the past decade. Single molecule imaging has witnessed similar growth, due to its capacity to reveal heterogeneities at high spatial and temporal resolutions. However, both resolution types are dependent on the signal to noise ratio (SNR) of the image. In this paper, we review how the SNR can be enhanced in optofluidics and microfluidics. Starting with optofluidics, we outline integrated photonic structures that increase the signal emitted by single chromophores and minimize the excitation volume. Turning then to microfluidics, we review the compatible functionalization strategies that reduce noise stemming from non-specific interactions and architectures that minimize bleaching and blinking.