Nanoparticles functionalized paper for biodiagnostic applications

Abstract

The research reported in this thesis focuses on the fundamental science and engineering required for the fabrication and optimization of nanoparticles functionalized paper as Surface Enhanced Raman Scattering (SERS) bio-diagnostics platform. It addresses the current Sensitivity, Selectivity, Simplicity and Strength (4S) limitations of bioactive paper. The research vision is to develop a nanoparticles treated paper as a generic diagnostic platform to identify and quantify low concentrations of a specific (bio)analyte in aqueous solutions. The product concept is a “dipstick sensor” based on functionalized gold nanoparticle treated paper (AuNP paper) for SERS detection. Such AuNP paper test would allow the direct, rapid and easy detection of (bio)analyte molecules at very low concentrations (below the nanomolar range), making it suitable for biomedical analysis, such as in cancer detection. It also eliminates the tedious preparation of multiple reactants and washing steps involved in current technology such as the Enzyme-Linked Immunosorbent Assay (ELISA) based bioactive paper. Three parallel research length scales are investigated: nano, micro and macro. This involves quantifying the distribution and adsorption state of nanoparticles on paper (nanoscale). The effects of gold nanoparticles (AuNPs) concentration and 3-dimensional (3D) distribution profile on their adsorption and aggregation states on paper were explored. The surface coverage of AuNPs on paper scaled linearly with their concentration profile in solutions. The SERS performances of the AuNPs-treated papers were evaluated with a model Raman molecule, 4-aminothiophenol (4-ATP), and their SERS intensities increased linearly with the density of AuNPs on paper. The role of z-distribution of AuNPs within the bulk of paper was highlighted; their three-dimensional (3D) architecture was able to quench the background noise through interlayer plasmon coupling, thus amplifying the SERS signal. The retention and aggregation state of nanoparticles on paper was controlled to drastically optimize the SERS performance (microscale). Paper substrates were pre-treated with a series of cationic polyacrylamide (CPAM) solutions to control the AuNPs adsorption and aggregation state on paper. The CPAM pre-treated paper produced a more uniform distribution of AuNPs compared to untreated paper. CPAM chains which adsorbed on paper in a high loops and tails conformation promotes efficient bridging of AuNPs to achieve higher surface coverage and aggregation of AuNPs on paper. This configuration is favored by CPAM solutions of higher concentration, charge density and molecular weight. A simple approach to form and visualize polyelectrolyte-nanoparticles chaplet like structures on paper was developed by using negatively-charged gold nanoparticles (AuNPs) to decorate polymer chain of CPAM. AuNP chaplets were adsorbed in an opposite direction to the cellulose fibers and along the length of the CPAM molecules which were draped over the fibers. The effects of CPAM polymer concentration, charge density and molecular weight on the dimension of AuNP chaplets were quantified. Assembling nanometer-scale components (AuNPs) into micrometer-scale arrays (chaplets) on a porous paper substrate can be a promising strategy for low-cost nanoelectronics applications The SERS properties of nanoparticles functionalized paper were quantified and optimized (macroscale). The effect of CPAM concentration, charge density and molecular weight on the aggregation and surface coverage of AuNPs on paper and ultimately the SERS signal of 4-ATP was quantified. The optimized AuNPs-CPAM paper showed a higher sensitivity and Raman enhancement factor (EF), which was almost an order of magnitude higher than the untreated AuNPs paper. The effect of CPAM dissolution kinetic on the SERS reproducibility was also quantified. SERS reproducibility of AuNPs on CPAM pre-treated paper both increase with dissolution time of CPAM, as the heterogeneity of AuNP distribution and aggregate size decreases; increasing CPAM charge density accentuated this tendency. After the SERS efficiency towards the detection of model Raman molecule (4-ATP) was proven, AuNPs paper was upgraded by functionalizing the AuNPs with biotin/streptavidin assemblies for the detection of antibody-antigen binding. The modification of antibody local structure due to the interaction with antigen was detected. Evidence of antigen binding was elucidated from the SERS spectra, confirming the presence of antigen. Reproducible spectra features were observed for the functionalized AuNP papers which were exposed to different concentration of antigen; their intensity increased as a function of antigen concentration. By conducting these studies and merging these three length scales of research, a novel strategy to engineer AuNPs functionalized paper as a low-cost and generic SERS platform for bio-diagnostic applications was developed.