Gold nanoparticle-based pH sensor in highly alkaline region at pH>11:: Surface-enhanced Raman scattering study

A surface plasmon resonance spectroscopy study showed that citrate-reduced gold nanoparticles (similar to 15 nm diameter, similar to 9 X 10(-9) M concentration, similar to 2 X 10(-2) M ionic strength) were found to be utilized as a colorimetric sensor by exhibiting a distinct color change at a highly alkaline pH > 11.5. Surface-enhanced Raman scattering (SERS) of 4-ethynylpyridine (4-EP) on gold nanoparticle surfaces indicated that the multiple peaks in the v(C equivalent to C) stretching bands should vary significantly in the highly alkaline region from PH 12 to 14. As the PH value increased, the v(C equivalent to C) stretching band intensity at similar to 2080 cm(-1) became stronger than that at similar to 2010 cm(-1). The pK(1/2) value was determined to be around 13 bv the SERS titration of taking intensity ratios of I-2080 with respect to I-2010. Using SERS enhancements and conspicuous spectral changes, selfassembled monolayers (SAMS) of 4-EP on An nanoparticles holds potential as a PH sensor for sensitive detection of the hydroxide OHconcentration at around PH 13 in an aqueous solution. The PH calibration from SERS titration of 4-EP is expected to have advantages in terms of higher alkaline detection limit and more precise measurements, if compared with the indigo carmine, the pK(1/2) value of which is 12.2.