Optimization of microfluidic functionalization of a plasmonic-based device for selective capture of anti-folic acid in solution

Recent advances in nanoscience and nanotechnology have led to the development of ultrasensitive plasmonic substrates with a theoretical limit of detection of a single molecule. However, the use of such devices in clinical practice and medicine is still hampered by their low selectivity and poor specificity. Even with the support of statistical data analysis methods, identifying specific biomarkers among hundreds of antagonists using an inherently label-free technology is still challenging. Here we develop a microfluidic device to optimize functionalization of the plasmonic substrate with a cysteine-folic acid complex for selective capture of anti-folic acid in solution. Since the latter is expressed by tumor cells, the device can potentially be used to detect and isolate circulating tumor cells from liquid biopsy samples. The plasmonic substrate is a 2D array of gold nanoparticle clusters fabricated by optical lithography and electrolytic deposition techniques. The system combines the sensitivity of a plasmonic substrate with the selectivity of a ligand recognition scheme to achieve identification of specific biomarkers in complex, low-abundance mixtures. The shape of the microfluidic chamber and the flow rate to be used were designed to functionalize the plasmonic substrate and to deliver biological solutions to the active sites of the device in an optimized and uniform manner. Fluorescence and SERS measurements were conducted on the functionalized samples. The results confirmed good and uniform immobilization and colocalization of receptors on plasmonic structures, enabling effective recognition of the target molecule.