Chromium is an essential metal in small quantities, but is toxic in larger quantities. For medical and environmental reasons, it may be important to determine how much of this metal ion is present in a sample.
Atomic absorption spectroscopy is the standard laboratory technique to selectively quantitate metal ion concentration in a solution. However, the instrumentation is expensive, and is typically not feasible for use in the field or in remote locations.
Yuqing Wu (State Key Laboratory of Supramolecular Structure and Materials, China) and coworkers have developed a cheap, easy-to-use, tailorable sensor for chromium ions in water. Their approach is selective for chromium over fifteen other metal ions, and cheap instrumentation can detect a concentration as little as 94 parts per billion.
The sensor.
The scientists coated gold nanoparticles with a molecule known as 5-thio-2-nitrobenzoate, which goes by the acronym TNBA. It possesses three relevant chemical units for the scientists' sensor.
One unit (known as the thiol unit) enables the molecule to chemically bind to the nanoparticles. Another two units (known as the carboxyl and nitryl units) enable the molecule to bind to chromium ions with a charge of +3.
An individual chromium ion can chemically bind to more than one molecule of TNBA. This causes the nanoparticles to clump together (aggregate) after 30 minutes.
It happens that the color of a solution of gold nanoparticles depends on the average diameter of the nanoparticles. Even if only a small amount of chromium ions are present in the solution, aggregating only a small number of the nanoparticles, the color change is easily detectable with cheap instrumentation.
The color change can be used to calculate the amount of chromium ions present in the sample. Thus, a color change tells one that chromium ions are present, and the quantity that is present.
Evaluation of the sensor.
The scientists found that gold nanoparticles coated with TNBA molecules are stabe at a pH higher than 6.0. At much less than that, however, the carboxyl unit loses its negative charge, which decreases the stability of the nanoparticles over time.
This is not a limitation. Most biological samples have a pH of approximately 7.4, well within the optimally useful range of the scientists' sensor.
Chromium metal ions (+3 charge) aggregated the nanoparticles. The extent of aggregation is readily calculated with cheap color-sensitive instrumentation.
Remember that the extent of aggregation is used to calculate the concentration of chromium ions in the sample. The 15 other metal ions the scientists tested generally did not aggregate the nanoparticles.
A minor exception is lead ions at an enhanced concentration, and iron ions which had an odd effect that is readily distinguishable from chromium-induced nanoparticle aggregation. The sensor is therefore selective for chromium ions.
Chromium ions were readily detected at a concentration of 94 parts per billion. The sensor is therefore sensitive to the presence of chromium ions.
Rejiggering the sensor for other applications.
The scientists next probed which chemical unit in their sensor, the carboxyl or nitryl unit, was most responsible for binding chromium ions. This will be important if the scientists are to rejigger the sensor for other purposes.
They found that chemically modifying the carboxyl unit eliminated the ability of the nanoparticles to aggregate in the presence of chromium ions. They further found that chemically modifying the nitryl unit enabled the sensor to aggregate in the presence of a wider range of metal ions, not just chromium.
Therefore, the carboxyl unit enables chromium ion binding, and the nitryl unit contributes to selectivity in metal ion binding. For further developing the sensor, it would seem wise to not tinker with the carboxyl unit.
However, it may be wise to tinker with the nitryl unit. With rational design, it is conceivable that the nanoparticles can be chemically modified such that they can serve as a sensor for different metal ions (those besides chromium).
Overall evaluation.
These scientists have developed a highly sensitive and selective sensor for chromium metal ions in water. The nanoparticles are easily fabricated; toxic ions are easily detected with cheap, portable instrumentation; and the nanoparticles can conceivably be retooled for detecting other metal ions besides chromium.
All of these characteristics suggest that their sensor shows great promise for a wide range of toxic metal ion assays.
for more information:
Dang, Y.-Q., Li, H.-W., Wang, B., Li, L. L., & Wu, Y. (2009). Selective Detection of Trace Cr3+ in Aqueous Solution by Using 5,5'-Dithiobis (2-Nitrobenzoic acid)-Modified Gold Nanoparticles
ACS Applied Materials & Interfaces, 1 (7), 1533-1538 DOI: 10.1021/am9001953