Malachite green is an organic compound known for its extensive use as a dyestuff. The use of malachite green has also been known for its controversial use as an antimicrobial in the aquaculture industry. The traditional use of malachite green has included its use as a dye material for silk, leather and paper. Although the name of the compound is malachite green it is not prepared from the malachite mineral and the name has been associated with it because of its intense green colour. The chemical formula of the compound Malachite green is C23H25ClN2 (chloride). The molecular weight of the compound which can be calculated from the molecular formula is 364.911 g/mol.
General Properties of Malachite Green
The structure of malachite green is given as follows:
[Image will be uploaded soon]
Further, the green coloured compound is given as :
[Image will be uploaded soon]
Malachite green is a chemical compound that is generally classified in the dyestuff industry. It is used as a triarylmethane dye and also extensively used in the pigment industry. In scientific terms, the name malachite green refers to the chloride salt [C6H5C(C6H4N(CH3)2)2]Cl. The term malachite green is normally used very loosely and commonly used in resemblance to the coloured green cation. Not only the chloride salt by the oxalate salt is also available in the market. The anions do not seem to have any effect on the colour of the compound. Usually, the intense green colours of the cation normally result from a very strong absorption band at 621 nm wavelength.
Malachite green is routinely prepared by the condensation of benzaldehyde and dimethylaniline, in turn giving leuco malachite green (LMG):
C6H5CHO + 2 C6H5N(CH3)2 → C6H5CH(C6H4N(CH3)2)2 + H2O
The leuco compound which is colourless relative to triphenylmethane is further oxidized to a cation which is the Malachite Green. The reaction is as given below:
C6H5CH(C6H4N(CH3)2)2 + HCl + 1⁄2 O2 → [C6H5C(C6H4N(CH3)2)2]Cl + H2O
The hydrolysis reaction of malachite green (MG) results in alcohol:
[C6H5C(C6H4N(CH3)2)2]Cl + H2O → C6H5C(OH)(C6H4N(CH3)2)2 + HCl
The alcohol produced here is important as it aids the malachite green to pass through the cell membranes. Once it is inside the cells, it is typically metabolized into the leuco malachite green. One thing to remember is that only the malachite green compound version is deeply coloured green. The other derivatives, the leuco and alcohol malachite green ones are not. This difference is because the cationic form of the compound extends a pi-delocalization due to which the molecule absorbs the visible light.
Preparation of Malachite Green
Hermann Fischer was the first one to prepare Leuco Malachite Green in 1877. He did so by condensing the benzaldehyde and dimethylaniline in the molecular ratio of 1:2. The reaction was carried out in the presence of sulphuric acid. The complete reaction is given below:
[Image will be uploaded soon]
Detection and Uses of Malachite Green
Malachite green is hard to detect. This is because it does not give out any fluorescence in an aqueous solution, a common method to detect fluorescence. Many research groups have developed different methods and techniques to detect malachite green. An example of this is the use of malachite green aptamer in microcantilever based sensors by Zhao et al. It is used to detect low concentrations of malachite green.
The most common use of malachite green since long has been its use as a dye. On average, kilotonnes of malachite green are produced along with other triarylmethane related dyes for their use as dye. It is also used as an active agent against the oomycete Saprolegnia, which is known to infect fish eggs, especially in commercial aquaculture. The use of malachite green is known for the treatment of Saprolegnia. This also extends its use as an antibacterial.
Along with being used against Saprolegnia, it is also used against Ichthyophthirius multifiliis that are found in the freshwater aquaria. Leuco malachite green is found as a principal malachite which is usually found in the samples of malachite green fish treatment. This has become the main cause of concern and so has become a basis for controversy and government regulations on the use of malachite green aquariums.
Malachite green over time has also been frequently and repeatedly used for the catching of thieves and pilferers. The bait, which in this case is usually money, is normally sprinkled with an anhydrous powder of malachite green. So, whenever any person comes in contact with the contaminated money will be caught as upon washing the hands, a green stain will appear on the skin which will last for several days.
All the applications of malachite green, even the niche ones, exploit the intense colour property of malachite green. As a result, it is used as a biological stain for the microscopic analysis of cell and tissue samples while studying cell biology. The Gimenez staining method uses basic fuchsin which stains the bacteria either red or magenta and so malachite green is then used as a blue-green counterstain. It is also used in the endospore staining as it can be readily used directly for staining the endospores present within the bacterial cells. In this particular case, safranin is used as a counterstain. Malachite green also forms part of Alexander’s pollen stains. It is further used for the saturable absorber in the dye lasers. It can also rarely be used for indicating the pH anywhere between 0.2 - 1.8.
In forensic sciences, leuco-malachite green is used as the detection method for latent blood. The haemoglobin in the blood catalyzes the reaction occurring between leuco malachite green and hydrogen peroxide. It converts colourless LMG into the malachite green. When there is an appearance of the green colour, it is indicative of the presence of blood in the sample.
Toxicity & Regulation of Malachite Green
In biological experimentation, rats are fed the malachite green drug LD50. The ones that have been fed with malachite green, experience an increase in liver DNA along with the lung adenomas. Leuco Malachite Green is also observed to cause an increase in the severity of the changes. As the LMG is the primary metabolite of malachite green found retained in the fish muscle for a longer time, the human intake of malachite green in the form of leuco malachite green increases which is harmful and has led to an increase in the more regulated use of malachite green. In biological experiments, rats were fed with 543 ppm of leuco malachite green.
Even though extreme in comparison to the possible human intake from fish of about 5 ppb, it was discovered that after a period of two years, the rats had an increase in lung adenomas in the male rats which is clearly a possibility for humans with continuous intake of malachite green. Hence, because of the carcinogenic symptoms, a malachite green application has come under governmental regulations, but there has been no case where any link between malachite green and liver tumour could be established.
Canadian authorities in 1992 determined that feeding fish having malachite green was a significant health risk. It was classified as a Class II Health Hazard. Still, it is used in many countries with less restrictive laws for anything other than commercial aquaculture because of the low manufacturing cost. Similarly, in 2005, analysts in Hong Kong found malachite green traces in eels and fish that were imported from China.
In 2206, the United States Food and Drug Administration (FDA) also detected malachite green in the seafood that was imported from China and other countries where the substance is banned for use in aquaculture purposes. Hence, in 2007, FDA blocked the import of many varieties of seafood that were having continued malachite green contamination. The substance has strictly been banned in the United States since 1983 and it has also been banned in the UK.
Thus, although malachite green has lots of biological experimentation applications, its consumption can be harmful and has been categorized as a health hazard.