Oriented assemblies of functional nanoparticles, with the aid of external physical and chemical driving forces, have been prepared on two-dimensional solid substrates. It is challengeable, however, to achieve three-dimensional assembly directly in solution, owing to thermal fluctuations and free diffusion. Here we describe the self-orientation of gold nanorods at an immiscible liquid interface (that is, oleic acid-water) and exploit this novel phenomenon to create a substrate-free interfacial liquid-state surface-enhanced Raman spectroscopy. Dark-field imaging and Raman scattering results reveal that gold nanorods spontaneously adopt a vertical orientation at an oleic acid-water interface in a stable trapping mode, which is in good agreement with simulation results. The spontaneous vertical alignment of gold nanorods at the interface allows one to accomplish significant additional amplification of the Raman signal, which is up to three to four orders of magnitude higher than that from a solution of randomly oriented gold nanorods.

In this paper, we report that an immiscible oil-water interface can achieve the dual function of electrokinetic molecular concentration without external electric fields and sensitive optical detection without a microscope. As a proof-of-concept, we have shown that the concentration of positively charged molecules at the oleic acid-water interface can be increased significantly simply by controlling the pH. Three-dimensional phase field simulation suggests that the concentration of positively charged rhodamine 6G can be increased by about 10-fold at the interface. Surface-enhanced Raman spectroscopy (SERS) is utilized for label-free detection by taking advantage of this molecular accumulation occurring at the interface, since gold nanorods can be spontaneously trapped at the interface via electrostatic interaction. SERS measurements suggest that the immiscible oleic acid-water interface allows the limit of detection to be improved by 1-3 orders of magnitude.

Transport of molecules and ions across cellular membranes is a fundamental process in biology. Visualization of such transport is an essential element toward a better understanding of how incoming molecules or ions interact with cellular membranes and diffuse into cytoplasm. However, detection techniques for this purpose have not been explored yet. Here we propose an innovative label-free detection technique that, in principle, permits real-time visualization of molecules across a cellular membrane. Lipid-modified gold nanorods (GNRs) in aqueous solution are exploited as nanoprobes for surface-enhanced Raman scattering (SERS)-based detection of neighboring molecules. For this purpose, the surfaces of as-synthesized GNRs were modified with a variety of phospholipids (DOPC, POPC, and DPPC, respectively) by extraction-based ligand exchange. The lipid-modified GNRs were characterized by transmission electron microscopy (TEM), UV-vis spectroscopy, and zeta potential measurement. As a proof-of-concept, SERS measurement of R6G with the lipid-modified GNRs was carried out. The limit of detection was found to be around 100 nM.