Optical Properties of Dopamine Molecules with Silver Nanoparticles as Surface-Enhanced Raman Scattering (SERS) Substrates at Different pH Conditions

Abstract

Silver nanoparticles (Ag NPs) prepared by the citrate reduction method were examined as surface-enhanced Raman scattering (SERS) substrates in the detection of dopamine (DA) molecules at different hydrogen ion concentrations. The aggregation of Ag NPs was influenced by the crosslinking effect of DA molecules as the function of pH of colloidal solution. Somewhat clustering of Ag NPs in a limited pH range (pH 7-9) exhibited the strong red-shift of absorption peak and maximal SERS activity to DA molecules, highlighting the importance of strong electrostatic adsorption and cross-linking effect that allowed DA molecules to reside in the junctions (hot spots) between aggregated Ag NPs. Furthermore, Ag NPs with DA molecules at strongly basic condition (> pH 9) exhibited the relatively high SERS activity as compared to negligible SERS activity at acidic condition (< or = pH 5), indicating the important role of oxidized surface of silver NPs which can interact with hydroxyl groups of DA molecules.

The characteristic AgcoreAushell nanoparticles as SERS substrates in detecting dopamine molecules at various pH ranges

“…The Ag core Au shell NPs with tunable plasmon resonances were prepared via an electron-less plating of Au layered over Ag colloids of different sizes (25,38, and 53 nm). The freshly prepared solution indicate the pH 5 and zeta potential of -15.43±7.32 mV.…”

“…25 The particle size was controlled by adjusting the concentration of citrate ions relative to the concentration of ionized silver complex in an aqueous solution. The Ag NPs (20-50 nm) were obtained by injecting aliquots (2-5 mL) of 1.0% sodium citrate (w/v) into 100 mL of boiling water containing 1.0% AgNO 3 (w/v in 1.0 mL) with vigorous stirring.…”

“…16,[21][22][23][24][25] The systematic investigation of the correlation between the aforementioned factors and the SERS activity of core-shell NPs is of great significance for the realization of highly efficient and reliable SERS substrates. Dopamine (DA) is a monoamine neurotransmitter in the catecholamine family, which plays a number of important physiological roles in animals.…”

“…32 It is generally known that the SERS signal is maximized when the excitation is conducted at a wavelength close to the plasmon resonance frequency. 33 As shown in Figure 3, the Ag core Au shell NPs (25 nm) with a plasmon peak at 516 nm exhibit much lower SERS activity than the Ag core Au shell NPs (38 or 53 nm) with more red-shifted Figure 1 The UV-vis spectra of the ag core au shell NPs with ag NPs with different core sizes (25,38, and 53 nm) at ph 5. The UV-vis spectra of singular ag NPs (38 nm) is included for comparison, in which the maximum peak position is located at approximately 400 nm.…”

“…In the following section, the core size of Ag NPs will be described by the number in parentheses, without expressing "Ag core size". Figure 3 demonstrates the SERS signals of the DA molecules adsorbed onto the Ag core Au shell NPs with different core sizes (25,38, and 53 nm) at pH 7. The band at ~1,480 cm -1 is assigned to the ring stretching vibration attributed to the stretching of the OC-CO bonds, and the band at 1,320 cm -1 is assigned to ring stretching vibration.…”