Surface-Enhanced Raman Scattering-Based Detection of Molecules in an Aqueous Solution via Lipid-Modified Gold Nanorods

Abstract

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.

Evaluating Phospholipid‐Functionalized Gold Nanorods for In Vivo Applications

“…A lower sonication time of 4 h resulted in AuNRs which aggregated during centrifugation, a sonication time of 24 h produced AuNRs which could be repeatedly pelleted by centrifugation without loss of the AuNRs’ plasmonic properties. After a single cycle of sonication, the ζ‐potential of DOPC AuNRs in 1 mM NaCl (pH 7) was measured to be positive (8 ± 4 mV), presumably due to the continued presence of CTAB (consistent with refs [ 39,42 ] ). After the second cycle of displacement, the ζ‐potential was measured to be neutral.…”

“…[ 38 ] They reported that displacing with DOPC alone led to unstable AuNRs, [ 38 ] which disagrees with other studies utilizing phosphatidylcholines. [ 39,40,42,49 ] We found here that increasing the sonication time to ≈24 h and undergoing three rounds of displacement produced stable DOPC coated AuNRs. Consequently, the protocol was also made more reliable when scaling up to several hundred milliliters.…”

“…While stability in biologically relevant media has been assessed for more common AuNR coatings such as CTAB and PSS, the stability of phospholipid-coated AuNRs outside of the undemanding environment of deionized water has received no attention. [40,42,48,66] The stability of our AuNRs was explored in a variety of "biologically relevant media", such as buffers, cell culture medium (CCM), serum, and plasma. Three buffers were selected as surrogates to test the impact of similar ion concentrations to those seen in human plasma on the stability of AuNPs.…”

“…This method has received some attention in the literature, although remains relatively uncommon. [38][39][40][41][42][43][44] These techniques generally consist of dispersing a pellet of CTAB-functionalized AuNRs in a solution containing a large excess of the desired surfactant and then providing energy to encourage exchange between the solution and the particle surface in the form of heat or sonication. These techniques have been reported for several molecules including phospholipids [38][39][40][41][42] and oleate.…”

“…[38][39][40][41][42][43][44] These techniques generally consist of dispersing a pellet of CTAB-functionalized AuNRs in a solution containing a large excess of the desired surfactant and then providing energy to encourage exchange between the solution and the particle surface in the form of heat or sonication. These techniques have been reported for several molecules including phospholipids [38][39][40][41][42] and oleate. [43,44] Phospholipids offer many benefits resulting from the highly tailorable properties of lipid membranes; including, low nonspecific binding, high biocompatibility, the easy conjugation of targeting ligands and fluorophores, and the inclusion of stericstabilizing agents such as PEG.…”