For the most part of human history, shiny and glittery gold has been valued because of its use in the making of coins, expensive jewellery, etc. Thanks to the field of nanoscience, or science of the tiny, which can make ordinary matter behave in extraordinary ways, invisibly small spheres of gold can potentially be lifesaving! These small metallic spheres are called gold nanoparticles (AuNP), and they can save lives by telling whether a sample of water is contaminated by poisonous heavy metals or not. Heavy metals like lead are known to be toxic as they can cause irreversible damage to the brain, and central nervous system. These ions can find a way to mix in streams of water due to human activities like mining. There exists a need to timely identify if the water which is being supplied to the public is contaminated by such toxins or not. There exist big, bulky, and expensive element identifiers that can do this job for you, but they are not widely accessible. One, therefore, has to come up with new, simple, and portable ways of testing the purity of water samples. Out of different identification mechanisms possible, methods utilising a change in solution colour to signal the purity of water sample are very attractive as they do not require the use of any sophisticated instruments. We, therefore, work with a solution of invisibly small, nano-sized spheres of gold in water, which looks unmistakably like a concentrated glass of wine! But how can such coloured solutions replace the use of element identifiers and signal the drinkability of water?
Gold Nanoparticles (AuNPs) have Positive and Negative charges on their surface
These coloured solutions of gold nanoparticles can be made to have either positive or negative charges at their periphery, depending on the kinds of molecules that are glued to their surfaces. Generally, when lead ions are added to a solution of negatively charged gold nanoparticles, the nanoparticles starts to form clumps, which ultimately precipitate out from the solution making the solution colourless. A solution, initially coloured and upon addition of lead, became colourless – this serves as an excellent way to identify lead ions! Interestingly, one gets a similar colour change if the water sample contains cadmium. Cadmium is lead’s evil twin cousin! It is evil because it can weaken your bones, and lead to a painful death. Differentiating cadmium from lead is not a trivial task, similar to the perplexing task of distinguishing between identical twins! Such negatively charged gold nanoparticles show a similar fading in colour when cadmium is present in water. So, one can therefore identify two toxic ions (lead and cadmium) using the same negatively charged gold nanoparticles. Why not use these particles as toxic ion sensors? These negatively charged nanoparticles, are bad, as they show the fading of colour in the presence of biologically relevant ions like calcium, magnesium as well. Moreover, what if, the water sample does not contain enough of the harmful ions, to decolourise the solution completely? What if, the colour changes are so small that one fails to notice it? Noticing the slight decrease in colour intensity is difficult, but slight appearances of colours are certainly noticeable! We, therefore, thought of making uncoloured solutions that show the appearance of colour to signal the presence of toxic ions. Another challenge in making such element identifying uncoloured solutions is that the colour should appear only in the presence of harmful ions, like lead. You would not want to throw water containing useful ions like magnesium, calcium, etc. just because your solution showed colour change! So how to get uncoloured solutions that satisfy these properties?
Negatively Charged Gold Nanoparticles show disappearance of colour in presence of both biologically useful ions like Calcium, Zinc, as well as toxic ions like Lead, Cadmium.
We used oppositely charged gold nanoparticles, which stick to each other because of strong electrostatic attractions and fall out as black precipitates giving rise to a colourless solution. These precipitates contain several millions of gold nanoparticles that are capable of making the colourless solution look like a glass of wine, once the electrostatic attraction breaks. This situation is analogous to that of a barrel, which is holding huge amounts of wine! We, then thought of using the abilities of different ions to break the interactions between nanoparticles (compromise the integrity of the barrel), leading to the liberation of a coloured solution of gold nanoparticles as the way to identify toxic heavy metal ions. The idea being, toxic heavy metal ions are extremely strong, and can compromise the stability of the precipitates easily, while the biologically relevant ions like calcium, and magnesium cannot. We found that out of all the ions tested, only lead was able to break this nanoparticle precipitates, leading to leaking of gold nanoparticles into the solution, making the solution wine red in colour! We repeated this same exercise with lead’s evil cousin cadmium, and found that cadmium could not show any colour change! Usually, it is extremely difficult to identify lead and cadmium apart. They are like identical twins, which are deadly! We found out that our strategy could differentiate between lead and cadmium, a not-so-useful but interesting observation! But an ideal sensor should be able to identify cadmium as well. So, by decreasing the strength of the electrostatic attractions between the nanoparticles, we could make sure that cadmium can break the interactions, and we can sense the presence of cadmium in drinking water. Interestingly, the good ions like calcium, magnesium, etc. still could not break the electrostatic interactions, and our system retained its ability to tell the presence of poisonous ions!
A mixture of Positively and Negatively charged AuNPs successfully showing a clear appearance of colour, only in the presence of toxic Lead
With this inexpensive, and scalable technology requiring minuscule amounts of positively, and negatively charged gold nanoparticles, anyone in any part of the world can check if the water available to her/ him is safe to drink or not. Moreover, we observed that two entities can come together to achieve a goal which was almost unachievable by either one of them independently.
This post is based on a recent work by our group. To know more, feel free to contact me.