Highly Selective Trace Analysis of Heavy Metal Ions in Water Using Aptamer-Conjugated Nanoparticles

Abstract

Heavy metal ions are highly toxic species which are known to disrupt biological events at cellular level, cause significant oxidative damage, and are carcinogens. Particularly, mercury is routinely released from coal-fired power plants, gold production, oceanic and volcanic emission and waste disposal. Mercury vapors and organic mercury derivatives, such as methylmercury, exert adverse effects on aquatic environment and subsequently human health. For instance, they can cause irreversible damage to the central nervous system and exposure of high mercury levels can be harmful to the brain and other organs of human of all ages, which is a problem of increasing social concern. The well-established analytical methods, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS), are powerful techniques for the determination of mercury, but they are time-consuming and laborious for on-site analysis. Accordingly, there is still growing demand to develop a simple, fast and low-cost technique for trace analysis of mercury(II) ions (Hg2+) in the environment, drinking water and aquatically derived food supplies. Surface-enhanced Raman scattering (SERS) spectroscopy has been shown ultra-sensitivity based on local field enhancement near certain metallic nanostructures. However, it is challenging for metal ions recognition using SERS technique since as a single atom, metal ions do not possess a vibrational spectrum and, therefore cannot be directly detected. Herein, we reported a new method for the trace analysis of Hg2+ in water. The approach involves the use of droplet-based microfluidics combined with surface-enhanced Raman scattering (SERS) detection. This novel combination provides both fast and sensitive detection of Hg2+ in water. Specifically, Hg2+ detection is performed by using the strong affinity between gold nanoparticles and Hg2+. This interaction causes a change in the SERS signal of the reporter molecule Rhodamine B (RB) that is a function of Hg2+ concentration. To allow both reproducible and quantitative analysis, aqueous samples are encapsulated within nanoliter-sized droplets. Manipulation of such droplets through winding microchannels affords rapid and efficient mixing of the contents. Additionally, memory effects, caused by the precipitation of nanoparticle aggregates on channel walls, are removed since the aqueous droplets are completely isolated by a continuous oil phase. Using this approach, quantitative analysis of Hg2+ with estimated limit of detection between 100 and 500 parts-per-trillion (ppt) was achieved. Compared with fluorescence-based methods for the trace analysis of Hg2+, the detection sensitivities were enhanced by approximately one order of magnitude. A novel sample approach was successively developed for the selective detection of Hg2+ using a silver nanoparticles (AgNPs) SERS sensing platform by employing a structure-switching signaling aptamer. The carboxytetramethylrhodmine (TAMRA) moiety-labeled single-strand DNA containing thymidine (T)-rich region is chosen as Hg2+ receptor where the TMARA acts as the Raman reporter. The aptamers electrostatically adsorb onto and cover AgNP surfaces by introducing spermine into silver colloidal solution. This results in slight aggregation concomitant with a weak SERS signal of TAMRA. Upon the addition of Hg2+, the aptamer changes the conformations to folded hairpin structure and form Hg2+-aptamer complex, leading to the exposure of AgNP surface without repulsion force and protection from aptamer. As a result, remarkable assembly of AgNPs caused by van der Waals interactions is observed and a significant increase of the SERS intensity of TAMRA is obtained. Under the optimum conditions, the facile and practical assay with a detection limit of 5 nM can be completed in less than 10 min. The high selectivity of this approach for Hg2+ analysis is also demonstrated. This proposed analytical method offers a rapid and reproducible trace detection capability for Hg2+ in water.