Exploration of Electrostatic Field Force in Surface-Enhanced Raman Scattering: An Experimental Investigation Aided by Density Functional Calculations

Surface chemical properties of metal nanoparticles must be tunable to create chemical specificity and are a key prerequisite for successful sensing and imaging platforms. To relate surface enhanced Raman scattering (SERS) to electrostatic field force, a simple colloidal chemistry approach has been deliberately exploited for syntheses of gold nanoparticles with negative and positive surface charges to study their interactions with charged analytes. We took up the challenge with sulfur-containing analytes because "Au−S" interaction is well-known. Thiocyanate ion, −SCN−, a well-known SERS analyte, has been proved to be chemically ligated/anchored on positively charged gold nanoparticles surface owing to favorable electrostatic attraction. The Au−S vibrational band at ∼240 cm−1 and blue-shifting of the −C≡N stretching frequency by ∼46 cm−1 in conjunction with its intensity enhancement by an order of ∼103 in the SERS spectrum clearly illustrate a chemisorption phenomenon. In contrast, physisorption of the −SCN− ion becomes evident on negatively charged colloid. Again, methylene blue has been shown to remain engrossed on the negatively charged gold surfaces. However, the electrostatic field force could not be accounted for from fluorescence quenching while methylaminopyrene was introduced because of the distance-dependence effect. The feasibility of such coordinative/chemical attachment also has been examined theoretically by density functional theory (DFT). Moreover, employment of this DFT calculation has been performed on five different metal−molecule interaction models to fruitfully interpret the experimental SERS findings and also the orientation of the SERS analyte. The observed Raman signals have been assigned from the potential energy distributions in terms of internal coordinates of adsorbate from the output of DFT calculations. The results thus provide a benchmark illustration of the value of DFT for aiding interpretation of adsorbate vibrational spectra attainable by using SERS.