NU 2016-114
INVENTORS Franz M. Geiger* Paul E. Ohno Hong Fei-Wang Kenneth B. Eisenthal SHORT DESCRIPTION A method to determine the charge of an interfacial surface using light
BACKGROUND The charge at an interfacial surface can be measured by the nondestructive optical technique called Sum Frequency Generation (SFG). As this technique is sensitive to solid/liquid interfaces, one of the primary uses of SFG spectroscopy in research and development applications is to look at the number and concentration of charges at the surface, where the accumulation and magnitude of positive or negative charges helps drive research in catalysis, biological and polymer membranes. While SFG has typically been used to determine the number and magnitude of charge at a surface, the typical output of traditional SFG is the square modulus of the true SFG signal (√(I^2 )), which does not allow for the observation of the sign of the charges, positive or negative. To overcome this, supplemental experiments such as streaming potential apparatuses or X-Ray photoelectron spectroscopy or without extensive alignment of a supplementary beam are performed, which are time-consuming and require costly instrumentation. Clearly, the use of detailed surface charge information is made much more valuable with the knowledge of the absolute sign of the charge.
ABSTRACT Northwestern researchers have used changes to the polarization of the light path to create an SFG apparatus that can observe the surface information such as charge density and magnitude along with the unambiguous sign of the surface charge. By modulating the polarization of light incident from the surface of interest the corresponding signal change can be related to the sign of the charges at the surface. The researchers demonstrate this in a proof-of-concept model, monitoring the change in negative charge on alpha-quartz/water interface upon the increase or decrease the pH. Outside of quartz, this technology is surface-general and can be used on any surface with known SFG signal. Moreover, the measurement is possible with any low-cost pulsed laser system and therefore offers significantly reduced cost for determining interfacial electrostatic parameters when compared to the method of common use, which is the streaming potential measurement apparatus.
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PUBLICATION Ohno P, Saslow S, Wang H-F, Geiger F, Eisenthal K (2016) Phase-referenced nonlinear spectroscopy of the α-quartz/water interface. Nature Communications 7: 13587.
IP STATUS Issued US Patent Nos. 10,274,807 and 10,514,585