Heavy metals have long been studied by many researchers across the world primarily due to its health-deteriorating effects when ingested by living organisms and the heavy burden it costs when released in the environment. When released in water, for example, biomagnification can cause exceedingly toxic levels of heavy metals to circulate across the food chain. While it is imperative that heavy metal release should be brought to a minimum, accidental releases, not to mention non-compliance to existing environmental regulations, caused the accumulation of heavy metals in different bodies of water.
Detection of heavy metals is the first among the series of steps that are needed to be done to eventually remediate these pollutants to acceptable concentrations.
However, existing detection methods such as inductively-coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS), while highly-accurate, are highly-expensive. Such methods require expensive instruments and trained operators before heavy metals can be quantified. Hence, there is a need to develop new methods to detect heavy metals in water without the need for expensive instrumentation
Low-cost point-of-need detection is important especially in rural areas who don’t have access to high-end laboratories.
Various methodologies have been developed in the past to address this concern. For example, electrochemical methods such as anodic stripping voltammetry and square wave voltammetry have been used for the detection of various heavy metals such as copper, lead, and mercury. Electrochemical methods are highly-reliable and can readily compete with AAS or ICP-MS. Nevertheless, it remains a challenge on how to lower the cost of sensing platforms without sacrificing the reliability and integrity of the sensor.
Our group is working on the use of nanoparticles as nanosensors for colorimetric detection of heavy metals. Colorimetry is relatively much simpler compared to existing detection methods.
In principle, the nanoparticles transmit a specific wavelength of light when it passes through its colloidal solution. This phenomenon is heavily reliant on the optical properties of nanoparticles. When an analyte is “sensed” by the nanosensor, the optical properties of the nanoparticles would change in some magnitude proportional to the concentration of the target analyte. This can be used as the “signal” for quantifying the concentration of heavy metals, and in general, is the general working principle of various colorimetric assays.
Nanoparticles can be tailored to detect a particular analyte. This can be done by functionalizing their surfaces with ligands that are specifically sensitive to the target analyte. Common mechanisms for sensing are complexation between the ligand and the analyte and the subsequent agglomeration of nanoparticles upon the addition of the analyte.
Recently, our group of undergraduate students presented their work on the colorimetric detection of copper ions in water by functionalizing silver nanoparticles at the Philippine Institute of Chemical Engineers Undergraduate Research Competition held at the De La Salle University, Manila last February 26, 2019. They bested the research presenters from other universities across the country.
Silver nanoparticles (AgNPs) exhibit localized surface plasmon resonance – the collective oscillation of electrons within the conduction band upon excitation with incident light. Such property is dependent on the size and shape of the nanoparticles. More importantly, functionalizing the nanoparticles allows us to fine-tune its applications to any target analyte that we wish to detect.
The use of nanoparticles as nanosensors is a promising field of research. We have high hopes that nanosensors can one day, compete, if not totally replace, the existing expensive instruments used for analyte detection.