Abstract
Single-molecule detection by surface-enhanced (resonance) Raman spectroscopy (SE(R)RS) relies on the production of single “hot spots” formed when metallic nanoparticles couple together to give a region of intense electromagnetic enhancement. Creation of aggregates by the addition of electrolytes to screen the Coulombic repulsion energy (Vmax) often leads to the formation of large aggregates, resulting in many “hot spots” within the laser probe volume. Controlled electrolyte addition can lower the repulsion to a value 0 < Vmax < 15 kBT leading to a solution which is enriched in small aggregates but does not show complete, uncontrolled aggregation. An important consideration is choosing an appropriate electrolyte to induce the aggregation, for example chloride anions displace the citrate layer on silver colloids preventing measurement of analytes with a binding affinity to silver more than citrate but less than chloride. Therefore, we have investigated the use of sulfate anions to prepare a colloid solution...
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