Controlling Gold Nanoparticles Aggregation in Continuous-Flow Microreactors for Robust Production of Ultrasensitive NIR SERS Nanotags

Surface-enhanced Raman scattering (SERS) nanotags are powerful tools for bioanalytical applications owing to their spectral specificity and detection sensitivity. Achieving strong SERS signals requires the generation of intense electromagnetic hot-spots, which are typically formed between closely spaced plasmonic nanoparticles, making the design of well-controlled nanoaggregates a promising approach for enhancing sensitivity in applications using SERS nanotags. This study demonstrates a flow chemistry approach for the controlled production of gold nanoaggregates optimized for surface-enhanced Raman scattering (SERS) under near-infrared (NIR) excitation. Using a two-stage microreactor system, 30 nm gold nanoparticles (Au NPs) are aggregated in a controlled manner with Malachite Green isothiocyanate (MGITC), acting simultaneously as Raman-reporter and aggregating agent, and then aggregation is rapidly quenched using HS-PEG-COOH or HS-PEG-folic acid (FA) to stabilize the nanoaggregates. Systematic variation of MGITC flow rate and reactor residence time enabled tunable aggregation, as confirmed via UV−vis spectroscopy, dynamic light scattering (DLS), transmission electron microscopy (TEM), and SERS. The flow setup also offered high reversibility, with on-demand switching between aggregation states during the reactor operation. The best-performing nanoaggregates provided over 100-fold SERS sensitivity improvement compared to nonaggregated Au NPs. Aggregation quenching with HS-PEG-FA offered additional targeting capabilities, enabling the complete production of nanoaggregates able to target cancer cells overexpressing folate receptors. In cancer tissue imaging experiments under 785 nm excitation, the nanoaggregates generated high-contrast SERS maps, while classical nonaggregated Au tags failed to generate detectable signals. Our platform offers on-demand production of tailored, ultrasensitive SERS tags suitable for biomedical imaging and diagnostics, combining enhanced signal strength and biological specificity.