Microimmunoassay using a protein chip: optimizing conditions for protein immobilization

Fabrication of micro-patterned substrates for plasmonic sensing by piezo-dispensing of colloidal nanoparticles

In this work we describe a very fast and flexible method for fabrication of plasmon-supporting substrates with micro-patterning capability, which is optimized for plasmonic sensing. We combined a wet chemistry approach to synthesize metallic nanoparticles with a piezo-dispensing system enabling deposition of nanoparticles on the substrates with micrometer precision. In this way, an arbitrary pattern consisting of 200 μm small spots containing plasmonic nanostructures can be produced. Patterns with various nanoparticles exhibiting different plasmonic properties were combined, and the surface density of the particles could be easily varied via their solution concentrations. We showed that under controlled conditions the dispensing process caused no aggregation of the particles and it enabled full transfer of the colloidal solutions onto the substrate. This is an important condition, which enables these substrates to be used for reliable plasmonic sensing based on monitoring the spectral shift of the nanoparticles. We demonstrated the functionality of such substrates by detection of small protein adsorption on the spots based on plasmon label-free sensing method.

Multiplexed immunosensing and kinetics monitoring in nanofluidic devices with highly enhanced target capture efficiency

Nanofluidic devices promise high reaction efficiency and fast kinetic responses due to the spatial constriction of transported biomolecules with confined molecular diffusion. However, parallel detection of multiple biomolecules, particularly proteins, in highly confined space remains challenging. This study integrates extended nanofluidics with embedded protein microarray to achieve multiplexed real-time biosensing and kinetics monitoring. Implementation of embedded standard-sized antibody microarray is attained by epoxy-silane surface modification and a room-temperature low-aspect-ratio bonding technique. An effective sample transport is achieved by electrokinetic pumping via electroosmotic flow. Through the nanoslit-based spatial confinement, the antigen-antibody binding reaction is enhanced with ∼100% efficiency and may be directly observed with fluorescence microscopy without the requirement of intermediate washing steps. The image-based data provide numerous spatially distributed reaction kinetic curves and are collectively modeled using a simple one-dimensional convection-reaction model. This study represents an integrated nanofluidic solution for real-time multiplexed immunosensing and kinetics monitoring, starting from device fabrication, protein immobilization, device bonding, sample transport, to data analysis at Péclet number less than 1.

Yeast surface display-based microfluidic immunoassay

In this paper, we present a new microfluidic immunoassay platform, which is based on the synergistic combination of the yeast surface display (YSD) technique and the microfluidic technology. Utilizing the YSD technique, antigens specific to the target antibody are displayed on the surface of engineered yeast cells with intracellular fluorescent proteins. The displayed antigens are then used for the detection of the target antibody, with the yeast cells as fluorescent labels. Multiplex immunoassay can be readily realized by using yeast cells expressing different intracellular fluorescent proteins to display different antigens. The implementation of this YSD-based immunoassay on the microfluidic platform eliminates the need for the bulky, complex and expensive flow cytometer. To improve the detection sensitivity and to eliminate the need for pumping, a functionalized micro pillar array (MPA) is incorporated in the microfluidic chip, resulting in a detection limit of 5 ng/mL (or 1 ng in terms of amount) and enhanced compatibility with practical applications such as clinical biopsy. This new platform has a high potential to be integrated into microfluidic detection systems to enable portable diagnostics in the future.

Impact of substrates for probe immobilization

Protein chips are becoming a key technology in proteomic research and medical diagnostics. Surface chemistry for immobilization of proteins forms the basis for assay design and determines the properties of protein microarrays. Optimal substrates provide a homogeneous environment for probes, preventing loss of biological activity and unspecific adsorption. Numerous immobilization approaches, based on covalent binding, affinity, or adsorption, have been proposed thus far, and these represent the toolbox for choosing optimized strategies for each individual application.