Simple preparation of positively charged silver nanoparticles for detection of anions by surface-enhanced Raman spectroscopy

Abstract

Modification of citrate and hydroxylamine reduced Ag colloids with thiocholine bromide, a thiol functionalized quaternary ammonium salt, creates particles where the zeta potential is switched from the normal values of ca. −50 mV to ca. +50 mV. These colloids are stable but can be aggregated with metal salts in much the same way as the parent colloids. They are excellent SERS substrates for detection of anionic targets since their positive zeta potentials promote adsorption of negatively charged ions. This is important because the vast majority of published SERS studies involve cationic or neutral targets. Moreover, the fact that the modifier is a quaternary ammonium ion means that the negative surface charge is maintained even at alkaline pH. The modified colloids can be used to detect compounds which cannot be detected using conventional negatively-charged citrate or hydroxylamine reduced metal nanoparticles, for example the detection limit was 5.0 × 10⁻⁵ M for perchlorate and <8.7 × 10⁻⁷ M for tetraphenylporphine tetrasulfonic acid (TPPS). More importantly, picric acid (an explosive) and diclofenac (a non-steroidal anti-inflammatory) could also be analysed quantitatively at low concentrations, 2.5 × 10⁻⁵ M and 1.9 × 10⁻⁵ M, respectively. Interestingly, the correct choice of aggregating agent is important for achieving high sensitivity since the anion in the aggregating salt may compete with anionic targets for surface binding sites. Finally, since the modification procedure simply involves reaction of nanoparticles with a small alkyl thiol derivative, it can easily be adapted to other particle morphologies or metals.