Metal nanoparticles for signal amplification in biosensors and signal suppression in fluorescence measurements

Metal nanoparticles are particles made up of many individual metal atoms ranging in size from one nanometer to hundreds of nanometers in diameter. Originally these particles were used in glazes to make lustrous patterns on pottery. It was not until Michael Faraday’s discovery of colloidal gold in 1856 that nanoparticles were characterized. Since then, nanoparticles of different compositions, sizes, and shapes have been implemented in fields such as catalysis and biotechnology. This dissertation will address two applications of nanoparticles: silver nanoparticles (AgNPs) to amplify signal in a paper-based fluidic device and gold nanoparticles (AuNPs) to quench signal in studies of cell penetrating peptides (CPPs). First, the paper-based device used concentrates a “sandwich-type” immunoassay, made of a capture antibody bound to a magnetic microbead and a detection antibody bound to a AgNP label, at a screen-printed carbon electrode by a magnet. The AgNP labels are oxidized to Ag ions, reduced, and stripped off the electrode in a technique known as anodic stripping voltammetry. Originally, this device could only detect AgNP concentrations as low as 2.1 pM. Scanning electron microscopy (SEM) was used to investigate the percentage of AgNPs oxidized in this detection method enabling optimization of the electrochemical parameters for higher signal output. This lowered the detection limit to 2.6 fM, which is now at a sensitivity comparable to a commercial pregnancy test. Second, this dissertation focuses on the use of AuNPs to suppress signal in fluorescence studies of cell penetrating peptides (CPPs), a class of peptides so effective in crossing cellular membranes that they can be used to carry cargo such as proteins or small molecules into a cell. The mechanism by which CPPs can permeate cell membranes is poorly understood. Here we use a model system composed of phospholipid vesicles and small fluorescent peptides to investigate the transport and mechanism of these peptides interacting with the lipid bilayer. While fluorescence signal can reveal details such as the amount of peptide that transports inside the vesicles, it cannot determine the depth that a peptide penetrates through the vesicles. By growing AuNPs inside these vesicles, however, we can use fluorescence quenching to determine the proximity of the peptide to the center of the vesicles, which will be important as we continue to study CPPs and their permeation through cell membranes.