Purification of gold nanoplates grown directly on surfaces for enhanced localized surface plasmon resonance biosensing.

Here we describe the synthesis and purification of Au nanoplates grown directly on surfaces by a chemical seed-mediated growth method. The synthesis involves the attachment of 3-5 nm diameter Au nanoparticle (NP) seeds onto glass and Si/SiOx surfaces and their subsequent growth into larger Au nanostructures by the chemical reduction of AuCl4- with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB). We used two different growth solutions. Growth solution 1 (GS1) led to a sample with 74% Au nanospheres and 26% Au nanoplates, while growth solution 2 (GS2), with lower CTAB and higher ascorbic acid concentration, led to 56% nanospheres and 44% nanoplates. The average wavelength of maximum extinction (lambdamax) of the localized surface plasmon resonance (LSPR) band of these samples was 549 and 627 nm, respectively. The use of adhesive tape or sonication enables the preferential removal of spherical Au nanostructures in both cases, leaving samples with >90% Au nanoplates. The average lambdamax increased to 672 nm (GS1) and 664 nm (GS2) for taped samples and 780 nm (GS1) and 720 nm (GS2) for sonicated samples, consistent with a higher purity of Au nanoplates on the surface. In all cases, the purified nanoplates vary in size and shape, including triangular, circular, or hexagonal structures, leading to broad spectra or the appearance of multiple peaks. We tuned the average lambdamax of the LSPR band of the Au nanoplate samples from 540 to 780 nm by varying the sonication time from 0 to 135 s. The change in lambdamax upon binding of anti-IgG to the edges of the purified nanoplates increases with an increasing number of anti-IgG on the edges, is 4-8 times larger compared to that of spherical nanoparticles, and is larger for samples purified by sonication compared to taping because the former has a larger initial lambdamax. A sample of Au nanoplates purified by taping and functionalized with anti-IgG at the edge sites displayed a shift in lambdamax as large as 45 nm for a 10 pg/mL solution of IgG (<1 pM).