An Update on the Use of Natural Pigments and Pigment Nanoparticle Adducts for Metal Detection Based on Colour Response

Abstract

Natural pigments occur in plants as secondary metabolites and have been used as safe colourants in food. Studies have reported that their unstable colour intensity might be related to metal ion interaction, which leads to the formation of metal–pigment complexes. This underlines the need for further investigations on the use of natural pigments in metal detection using colorimetric methods, since metals are important elements and can be hazardous when present in large amounts. This review aimed to discuss the use of natural pigments (mainly betalains, anthocyanins, curcuminoids, carotenoids, and chlorophyll) as reagents for portable metal detection based on their limits of detection, to determine which pigment is best for certain metals. Colorimetric-related articles over the last decade were gathered, including those involving methodological modifications, sensor developments, and a general overview. When considering sensitivity and portability, the results revealed that betalains are best applied for copper, using a smartphone-assisted sensor; curcuminoids are best applied for lead, using a curcumin nanofiber; and anthocyanin is best applied for mercury, using anthocyanin hydrogel. This provides a new perspective on the use of colour instability for the detection of metals with modern sensor developments. In addition, a coloured sheet representing metal concentrations may be useful as a standard to support on-site detection with trials on masking agents to improve selectivity.

An Update in Computational Methods for Environmental Monitoring: Theoretical Evaluation of the Molecular and Electronic Structures of Natural Pigment–Metal Complexes

“…Hydration can impact the physicochemical properties of metals, pigments, and metal-pigment complexes; the interactions that occur; and the reactivity and stability of various systems. In addition, water molecules can impact the compounds and the complexes formed in diverse ways [39,111].…”

“…At present, environmental pollution by metals, especially heavy metals, is an unavoidable threat and challenge, especially with increasing industrial activity without proper waste processing [31][32][33][34][35] and an increase in traffic volume [36]. Pollution spreads through the air, water, animal products, agricultural products, and various other components into the surroundings [37][38][39][40]. Metals can enter plants, animals, and humans in various ways, such as through the respiratory tract, food and drink, and even skin contact.…”

“…These methods show good selectivity and sensitivity; however, they lack efficiency when viewed in terms of time, energy, and costs. Moreover, the availability of instruments is still limited [20,39]. Due to existing limitations, the development of portable detectors that are efficient in terms of time, cost, and energy could represent a solution.…”

“…Computational calculations will be very useful in predicting the possibility of colour shifts because changes in the ground-state geometry will affect the absorption peak shift [66]. A small change in the basic geometry of a molecule can trigger larger shape and chemical changes, including optical properties, distribution of electrons, and electronic structure, that shift the wavelengths of light it absorbs, which is directly related to the colour change that can be observed by the naked eye [39,67,68]. Until now, there has been no review that discusses this field, even though this preliminary research is very important and useful.…”

Novel Applications of Silk Proteins Based on Their Interactions with Metal Ions

“…Aside from metal ions, silk proteins can also combine with natural nonmetallic materials to form biosensing composites. For instance, silk protein/anthocyanin composites can be used to detect metal ions in aqueous solutions and show the saturation state of metal ion adsorption by the composite [96,[211][212][213]. This is because anthocyanin, a natural water-soluble pigment, exhibits significant color changes upon interaction with metal ions.…”