Frontiers in Diagnostics

Magnetism: Giant MagnetoResistors (GMRs)
and Magnetic Materials

Magnetism can be used in novel ways for sample manipulation, device actuation, detection, and readout for assays in either a microfluidic construct or a standard planar format. The read/write heads used in personal computers today employ giant magneto-resistors (GMRs) capable of reading a 0.0065-?m2 bit at 300 Mb/s, which, when optimized for use in bio-analyses, translates into an extremely powerful means to readout massively multi-plexed sensor arrays. To this end, we have used GMRs with various magnetic labeling strategies for the detection of protein-protein interactions and sandwich immunoassays. Other practical applications of magnetism in miniaturized architectures that we are exploring include fluid actuation devices (i.e., pumps and mixers), sample and label diverters, and flow detectors.

 

Nanoparticle Synthesis

The novel properties, minute size, large surface area-to-volume ratio, and ability to be uptaken by bioentities all make nanoparticles of immense interest in our group. As noble metals (e.g., Au) are reduced in size to several tens of nanometers a new energy absorption band is observed - the surface plasmon oscillation - which makes these nanoparticles attractive spectroscopic labels (e.g., Extrinsic Raman Labels, ERLs). Further, through shape manipulation by molecularly driven atomic assembly, we can alter the surface Plasmon oscillation of these particles. To understand and use these novel nanostructures effectively we strive to characterize both their general and interfacial properties.

 

Surface Enhanced Raman Scattering (SERS)

As a readout tool for biological assays, SERS has excellent limits of detection approaching attomolar levels (10-18M) and offers rapid analyses. Additionally, it offers promising multiplexing strategies as Raman bands are 10-100 times narrower than fluorescence, which allows the use of many more reporter labels. SERS hardware is simple and rugged, using a single excitation source for each substrate. Both hardware and assay are not sensitive to humidity or affected by quenchers, facilitating field deployment. SERS also lends itself to multi-purpose addressing since the label acts as both identifier and quantifier, and we can take advantage of the nanoparticle shape to aid in label discrimination. To date we have demonstrated the efficacy of this strategy to detect low levels of cancer markers, biowarfare simulants, viruses, and bacteria with a total workup and readout time of less than an hour.

 

Scanning Probe Microscopies (SPM)

The suite of SPM modes includes Atomic Force Microscopy (AFM), Friction Force Microscopy (FFM), Electric Force Microscopy (EFM), and Magnetic Force Microscopy (MFM) among others. These instruments can be used to map disparities in surface composition, modify surfaces at the atomic level, and glean information of surface-bound chemical substructures. The nanometric resolution of these techniques makes visualization of surfaces modified with biological materials (e.g., DNA, viruses, bacteria, etc.) possible. Our goal is to extend this visualization to enable interrogation of sandwich assays through height based readout. Our initial studies in this area have focused on quantifying porcine parvovirus and feline calcivirus incubated on capture substrates. Currently we can detect 104 to 106 viruses/mL using this technique.

 

Transduction Amplification, Hydrodynamics, and Fluidic Control

Signal amplification through electrochemical recycling is an efficient means to monitor analyte signatures throughout the course of a flowing or stagnant experiment and can result in amplification factors of 102. We typically incorporate fluid flow in experimental designs that require an active form of mass transport, i.e., where diffusional mass transport may limit one or more of the experimental figures of merit. We have begun exploring the impact that various forms of convective mass transport have on assays. (e.g., decrease in non-specific binding, increase the assay rapidity, lowering limits of detection, etc.). Our studies involve not only overall assay conditions but also encompass sample injection and fluid focusing schemes for micro-miniaturized assay platforms, i.e., lab-on-a-chip architectures and micro-total-analysis systems (mTAS).