Abstract
Charge density and molecular coverage on the surface of electrode play major roles in the science and technology of surface chemistry and biochemical sensing. However, there has been no easy and direct method to characterize these quantities. By extending the method of Transient Induced Molecular Electronic Signal (TIMES) which we have used to measure molecular interactions, we are able to quantify the amount of charge in the double layers at the solution/electrode interface for different buffer strengths, buffer types, and pH values. Most uniquely, such capabilities can be applied to study surface coverage of immobilized molecules. As an example, we have measured the surface coverage for thiol-modified single-strand deoxyribonucleic acid (ssDNA) as anchored probe and 6-Mercapto-1-hexanol (MCH) as blocking agent on the platinum surface. Through these experiments, we demonstrate that TIMES offers a simple and accurate method to quantify surface charge and coverage of molecules on a metal surface, as an enabling tool for studies of surface properties and surface functionalization for biochemical sensing and reactions.
Highlights
Because of the importance of surface charge density to surface chemistry, extensive molecular dynamic simulations have been performed to calculate the surface charge density for solutions of different ionic composition and pH value[3,4]; and many attempts have been made to experimentally measure this and other related quantities[5,6]
The Transient Induced Molecular Electronic Signal (TIMES) system consists of a microfluidic device with two parallel microfluidic channels that are connected to a single channel via a Y-junction
TIMES method has been proved to be capable of measuring surface charge density with high signal quality
Summary
Because of the importance of surface charge density to surface chemistry, extensive molecular dynamic simulations have been performed to calculate the surface charge density for solutions of different ionic composition and pH value[3,4]; and many attempts have been made to experimentally measure this and other related quantities[5,6]. Atomic force microscopy (AFM)[7,8,9,10,11], surface plasmonic resonance[12,13], streaming potential[14,15,16,17,18], and contact angle titration[19,20,21] are among the most studied techniques that can produce information related to the surface charge density none of the existing methods, to our best knowledge, can and directly measure the polarity and amount of surface charge in the natural environment where surface reactions take place. If a molecule, charged or not, is permanently anchored to the surface of electrode, it would not contribute to the TIMES signal. This is another salient feature for our technique since the knowledge of area coverage of certain molecules such as deoxyribonucleic acid (DNA) capture probes or antibodies is valuable for optimization of the reaction conditions and ensuring reproducible results for any microarrays
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