A-394 Scalable and continuous production of monodispersed gold nanospheres capped by citrate species for advanced lateral flow immunoassays

Abstract Background Lateral flow immunoassays (LFI) based on gold nanoparticles have emerged as a reliable tool for point-of-care applications. The conventional particles prepared using the Turkevich method are poorly defined in shape and size, making them unsuitable for advanced analysis. Previously, we had developed a method based upon seed-mediated growth for the synthesis of gold nanoparticles with a perfectly spherical shape. The nanospheres exhibit the narrowest optical absorption peak, making them ideal for multiplexing analysis. However, the nanospheres are capped by cetyltrimethylammonium chloride/bromide (CTAC/B), toxic ligands that also impede the binding of analytes. In general, the formation of nanospheres requires a dedicated balance between the atom deposition and surface diffusion rate. As such, dropwise addition of the precursor must be used, limiting the scale of production needed for commercial application. Here, we address these limitations by demonstrating a scalable and continuous method for the synthesis of citrate-capped gold nanospheres for advanced lateral flow immunoassays. Methods The synthesis comprises two steps. Firstly, we developed a scalable method for the synthesis of gold nanocubes with uniform edges length by introducing the precursor in one shot, followed by incubation at an elevated temperature to transform the shape from cubic to spherical. Secondly, we used a simple method to exchange the CTAC/B on the nanospheres with citrate species. It involves the deposition of an ultrathin shell of fresh gold on the nanospheres in the presence of citrate, which can serve as a reducing agent for the precursor and a ligand for further surface modification. Results The gold nanospheres synthesized in the first step have a monodispersed size from 10-35 nm. Significantly, our recent study demonstrates that this protocol can be extended to a flow reactor with a throughput of 5 × 10?6 moles of Au per minute, achieving a tenfold increase over the batch process based on the Turkevich method. It enables the large-scale production of gold colloids to meet the requirements for commercial application. In the second step, the surface-bound CTAC/B are replaced with citrate species, as confirmed by Fourier-transform infrared spectroscopy. During gold deposition, the CTAC/B desorbs while citrate species adsorb on the surface. Ultraviolet–visible spectroscopy further confirms that colloidal stability is retained after ligand exchange. In addition, our recent study demonstrates that this approach is also adaptable to a flow reactor for scalable operation. Conclusion We have developed methods for the continuous and scalable production of monodispersed gold nanospheres sought for use as color markers in diagnostic applications such as LFI. The surface of the nanospheres can be made with citrate to match that of the conventional gold colloids. As such, one can directly incorporate the gold nanospheres into current commercial devices to achieve new capabilities such as enhanced sensitivity and quantitative analysis. The successful transformation of the two-step synthesis methodology into a continuous flow reactor production process demonstrates the robustness of the innovation for commercial-scale industrial use. The transition from batch to continuous synthesis represents a step change in large-scale manufacturing simplicity, cost and quality.