Metal/dendrimer nanocomposites for enhanced optical breakdown: Acoustic characterization and initial targeted cell uptake study

Metal/dendrimer nanocomposites (DNCs) uniquely combine the properties of metallic clusters and the biofriendly polymer host in a nanosized hybrid particle. DNCs can biochemically target tissues and locally reduce femtosecond optical breakdown thresholds, making highly precise and selective photodisruption possible. In this study, we have used high-frequency acoustic monitoring of bubble production dynamics to investigate how DNC properties, solution concentration, and optical parameters affect threshold reduction, actual waiting time, and mechanical characteristics of breakdown. Breakdown is defined here as bubble production with an onset of less than 20 seconds after laser exposure. DNC properties varied include metal content (silver, gold) and terminal group (amino-NH₂, glycidol-OH, and carboxylCOOH) which determine pH values. Results indicate that DNC metal content markedly influences solution threshold reduction, while DNC terminal group (and thus net surface charge) and solution concentration influence the details of breakdown at these reduced threshold fluences. (Ag(0)} DNCs reduce breakdown threshold fluence 1-2 orders of magnitude more than (Au(0)} DNCs. Furthermore, concentrated DNC solutions and DNCs carrying a net negative charge (carboxyl terminal groups) increase bubble production up to four times and shorten waiting time for breakdown from seconds to milliseconds. Increasing laser fluence for a given DNC solution concentration also shortens breakdown waiting time. Lastly, utilizing the fluorescence properties of silver nanocomposites, we use confocal microscopy to examine KB cell uptake of folate targeted silver DNCs. Cells incubated with folate targeted silver DNCs exhibit a measurable increase of intracellular fluorescence compared to control cells (no DNC incubation). However, while we observe a threshold reduction in KB cells incubated with 500nM folate-targeted DNC solution, there is no threshold reduction in cells incubated with 50nM folate-targeted DNC solution. This suggests that a specific minimum DNC concentration may be required for localized reduced-threshold breakdown to occur.