Noble metal nanoparticles - Au and Ag - for biodetection

Dissertation submitted for obtainment of the Master’s Degree in Biotechnology, by the Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

Metal nanoparticles possess unique optical, chemical and magnetic properties due to their size, shape and composition. Taking advantage of these properties, new biosensors have been developed using, mainly, gold nanoparticles. Silver nanoparticles, due to its enhanced surface plasmon resonance extinction coefficient are alternate candidates as labels to biodetection. However, unlike gold nanoparticles, silver nanoparticle derivatization with thiol-modified oligonucleotides requires cumbersome and time-consuming protocols. To circumvent this limitation, an approach is the use of gold-silver alloy nanoparticles, taking advantage of the ease of derivatization of gold nanoparticles and the enhanced surface plasmon resonance extinction coefficient of silver nanoparticles. This work describes the synthesis and characterization of gold-silver alloy nanoparticles (50% gold, 50% silver) and their thiol-ssDNA functionalized counterparts (nanoprobes) for application in molecular diagnostics. These new nanoprobes were used to specifically detect a sequence derived from the RNA polymerase -subunit gene of Mycobacterium tuberculosis, the etiologic agent of human tuberculosis. Complementary targets were detected using a non-cross-linking assay that consists on the spectrophotometric comparison between solutions before and after salt-induced nanoprobe aggregation. This new approach should allow the use of gold-silver alloy nanoparticles with different gold molar fractions, or even bimetallic nanoparticles composed of other metals (e.g., Cu, Pt) in the development of biosensors. The conjugation of these new nanoprobes with the well-established gold nanoparticle system can be the basis of new multiplex methods for specific DNA, RNA and/or other molecules biodetection.