Paper-based sensors and assays have been highly attractive for numerous biological applications, including rapid diagnostics and assays for disease detection, food safety, and clinical care. In particular, the paper immunoassay has helped drive many applications in global health due to its low cost and simplicity of operation. This review is aimed at examining the fundamentals of the technology, as well as different implementations of paper-based assays and discuss novel strategies for improving their sensitivity, performance, or enabling new capabilities. These innovations can be categorized into using unique nanoparticle materials and structures for detection via different techniques, novel biological species for recognizing biomarkers, or innovative device design and/or architecture.

The unique size and material dependent properties of nanoparticles have made them highly attractive for biological and medical applications. However, combining nanoparticles with biomolecules and biological environments has faced many challenges. These interface issues often involve protein denaturation, steric hindrance, and orientational issues for the biomolecule, which can impair function and decrease overall performance of the nanoparticle-biomolecule conjugate. Historically, our understanding of the physical and chemical properties of nanoparticle-biomolecule conjugates as appropriate tools and experimental techniques had to be determined. We discuss here selected examples investigating the fundamental physical properties of the interface between nanoparticles and DNA and proteins and protein coronas and how they have provided insight into the properties of the biomolecule when it is interfaced to a nanoparticle.

We report a quantitative evaluation of the choice of reporters for multiplexed surface-enhanced Raman spectroscopy (SERS). An initial library consisted of 15 reporter molecules that included commonly used Raman dyes, thiolated reporters, and other small molecules. We used a correlation matrix to downselect Raman reporters from the library to choose five candidates: 1,2-bis(4-pyridyl)ethylene, 4-mercaptobenzoic acid, 3,5-dichlorobenzenthiol, pentachlorothiophenol, and 5,5′-dithiobis(2-nitrobenzoic acid). We evaluated the ability to distinguish the five SERS reporters in a dipstick immunoassay for the biomarker human IgG. Raman nanotags, or gold nanostars conjugated to the five reporters and anti-human IgG polyclonal antibodies were constructed. A linear discriminant analysis approach was used to evaluate the separation of the nanotag spectra in mixtures of fixed ratios.

Surface enhanced Raman spectroscopy (SERS) has been attractive for enhancing the sensitivity of lateral flow immunoassays (LFA). A format that has enabled specific detection of biomarkers is to use Raman reporter molecules linked to gold nanoparticles (NPs), which are conjugated to antibodies specific for the target of interest. Many factors such as the NP and Ab properties and the method of signal readout impact the sensitivity of a SERS based immunoassay. To understand how to optimize assay sensitivity, we studied SERS readouts of multiplexed sandwich immunoassays for the zika and dengue non-structural protein 1 (NS1) biomarkers as a test case. We investigated the effect of NP shape on the SERS enhancement of the reporter molecules 1,2-bis(4-pyridyl)ethylene (BPE) and 4-mercaptobenzoic acid (MBA). We also performed SERS imaging of test lines to map the spatial distribution of signal in test lines on the nitrocellulose. Finally, we used a modified least squares analysis to differentiate reporter contributions.