The mercaptooctanoic acid coated hydrophilic gold nanoparticles, with diameter ranging 2-5 nm verified from TEM, showing strong emission at 610 nm with UV excitation has been previously developed in our group. The pH sensitivity, reversibility and stability of the nanoparticle were studied and possible explanations are described in this paper.

At higher pH, the carboxylic acid groups will be deprotonated thus the surface of the gold nanoparticle will be negatively charged. The charge-charge repulsion becomes greater than hydrophobic interaction of octanoic acid chain which makes the particles separated and well dispersed. Meanwhile, at lower pH, the carboxylic groups at the surface of the nanoparticle will be protonated and neutralized. With the neutralized charge at the end of the capping molecule, hydrophobic interaction dominates which eventually results the aggregation of the particles. This aggregated particle cluster can block and scatter the excitation as well as emission lights to/from the individual nanoparticle so that the emission intensity attenuates.

Also, at the charged state, a pair of electrons will occupy high energy π anti-bonding molecular orbital while they will occupy low energy σ molecular orbital formed between oxygen and hydrogen atoms. This indicates that at the charged state, it could be easier for those electrons occupying higher-energy molecular orbital to be excited to reach conduction band which results in brighter emission with charged state comparing to the neutralized state of the ligand molecules.

The fluorescence intensity of the gold nanoparticle has been decreased, up to 32%, relative to its highest intensity at higher pH. Also, the change of the fluorescence intensity is reversible to the pH change which opens up the possibility of the nano-sensor application such as encapsulating the gold nanoparticle inside the transparent nanoshell to monitor the pH of nano-environment. The pH stability test was performed in 8 cycles of changing in the range of the physiological pH of 8 to 5.5.

We have shown the pH sensitivity, reversibility and stability of the fluorescence of gold nanoparticles. It can be applied to develop a nano-pH sensing probe for various applications including intracellular pH monitoring.