Selective Manipulation Of Molecules By Electrostatic Force And Detection Of Single Molecules In Aqueous Solution

Manipulation of biomolecules in aqueous solution has been a critical issue for the development of many biosensing techniques and biomedical devices. Electrostatic force is an effective method for increasing both sensitivity and selectivity of various biosensing techniques. In this study, we employed surface-enhanced Raman spectroscopy (SERS) as an in situ label-free method to monitor the motion of biomolecules driven by this manipulation technique. We present the results of a combined experimental and simulation study to demonstrate that electrostatic force could enhance SERS detection of molecules in aqueous solutions with respect to sensitivity and selectivity. In regards to sensitivity, we successfully observed the signature of single molecule addition to individual SERS hot spots, in the form of the stepwise increase of Raman signal with time. With regard to selectivity, we obtained discernible SERS signature of selected families of molecules from a mixture of other molecular families of higher concentration by driving the specifically charged or polarized molecules toward or away from the electrodes/SERS surface based on their charge state, polarizability, mass, and environment pH value. We further report the experimental results on how the key factors affect the selective attraction and repulsion motion of biomolecules.