The present state-of-the-art for groundwater monitoring relies heavily on withdrawal of water samples for subsequent laboratory analysis. This process is time consuming and operator-intensive. The use of in situ sensors would obviate much of the sample manipulation and on-site labor required. The objective of this project was to establish proof-of-principle for a new class of sensors for monitoring toxic metal ions in groundwater utilizing nanoengineered, highly selective, high surface area sorbent materials.
This project consisted of three tasks. First, functionalized mesoporous silica-based sorbent materials were embedded in conductive carbon paste, and thiol functionalized mesoporous silica (SH-FMS) thin films were applied to the faces of electrodes to create the Thiol-Self-Assembled Monolayer on Mesoporous Silica (SH-SAMMS). Secondly, the SAMMS were used to perform square wave anodic stripping voltammetry (SWASV) to demonstrate detection of Pb2+ and Hg2+ from aqueous solution. Finally, the feasibility of depositing the functionalized mesoporous silica thin films to interdigitated electrodes was examined, to determine whether adsorption of metal ions from solution would create a capacitive signal, thereby providing proof of principle of another class of sensor based on functionalized mesoporous silica sorbents.
It was found that the SH-SAMMS modified carbon paste electrode coupled with an anodic stripping voltammetry technique could be used to detect lead (II) and mercury (II) simultaneously. The SH-SAMMS modified carbon paste electrode had many advantages over the existing chemically modified electrodes, which normally use commercially-available ligands as the modifiers. The high surface area of the mesoporous silica and the covalent binding between the thiol (-SH) groups in SH-SAMMS and metal ions in solution made SAMMS a better sorbent than existing commercial ligands. The SH-SAMMS were shown to be dramatically faster and more selective for adsorption of Hg, with the selectivity a factor of 1000 higher than that of DuoliterTM GT-73, an organic ion exchange resin commonly used for Hg removal.
The selectivity of SH-SAMMS to the target metal ions (Hg and Pb) also was notable, as SAMMS did not accumulate common metal ions such as sodium and calcium, which are often present in wastewaters at much higher concentrations than the target ions. The high loading capacity and high selectivity of SH-SAMMS are desirable for metal ion detection because they minimize the competition for the binding sites of the non-target species, thereby reducing the interferences and preserving the signal intensity of the target metal ions. Additionally, since SAMMS particles are not conductive, the high surface of the material did not contribute to the charging current of the SAMMS modified carbon paste electrode.
The binding between SH-SAMMS and metal ions was reversible, therefore the SH-SAMMS based electrodes could be easily regenerated without damaging the ligand monolayer by desorption of the preconcentrated species in an acidic solution. By choosing an appropriate acid solution as a stripping medium, the electrodes were, usually, ready for reuse after the stripping voltammetric measurement. The special advantage of using SAMMS over other organic ligands is that de-aeration of the sample, electrolysis medium, and stripping medium is not required, making the SAMMS-modified electrodes suitable for integration into portable sensor devices for on-site metal detection. In addition, self-assembled monolayer chemistry readily allows installation of a wide variety of chemical monolayers that can be tailored to specific metal ion detection needs when used as electrode modifiers.
This project demonstrated that these electrodes have potential to be a promising selective detection technology for monitoring toxic metal ions in groundwater. The selectivity of the SAMMS technology coupled with the rapid collection and detection methods afforded by the SWASV technique represent a clear step forward in the development of devices for rapid, fieldable devices for environmental remediation and testing for adherence to water quality standards.