Recent advances in chemically and biologically synthesized nanostructures for colorimetric detection of heavy metal

3.1 Heavy metal detection using Eco-friendly green synthetic approaches

Nanomaterials synthesize by green synthetic strategies are not hazardous for the environment as compare to the conventional methods being used up previously in labs and industries. Green synthesis methods have been involved in utilization of essential phytochemicals e.g. carboxylic acids, amides, phenols, ketones, aldehydes, flavonoids, alkaloids, ascorbic acids and various other biological components (fungi, plants, bacteria, and yeast) as reductants or protective agents (Narayanan and Sakthivel, 2011; Yurkov et al., 2011; Marchiol, 2012; Iravani, 2014; Singh et al., 2018; Azmi et al., 2021). There are different natural backgrounds on which green synthesis depends along with several reaction conditions as solvent system pH and temp. For fabrication of metals nanoparticles, phytochemicals are supposed to be the most suitable option. A green and reliable method in which extracts of lemon and oranges were used have been by reported (Ravi et al., 2013). Lemon and orange extract is rich source of ascorbic and citric acid and capable of reducing the Ag and Au ion into the simple metal nanoparticles. Although, lemon and orange are considered to be the good reducing agents, but extract of lemon under the exposure of sunlight work as reducing agent for silver nanoparticles. While orange extract act as stronger reducing agent even in absence of sunlight. Instrumental studies make sure that Au and Ag NPs based on the extract of orange and lemon were more sensitive towards harmful Hg²⁺ at the micro-molar level and sensed Hg²⁺ at varied pH range (3.2–8.5). The involvement of eucalyptus globules leaf extract has also been reported (Madhavi et al., 2013; Annadhasan et al., 2014). Batch experiment was used under the controlled adsorption factor at room temperature for the removal of Cr⁶⁺ by zero-valent-iron nanoparticles (ZVNI) and results 98.1% adsorption capacity at dosage rate of 0.8 g/L within 30 min. However, it is important to note that stability period after synthesis was found two months for the as-prepared Fe-NPs. Moreover, Annadhasan and his colleagues have reported sensing of Pb²⁺, Hg²⁺, and Mn²⁺ metal ions in aqueous media by using L-tyrosine stabilized Au and Ag NPs under optimized conditions (Annadhasan et al., 2014). It has been found that the presence of OH and –COO^– functional groups act as strong reducing agents for reduction of metal ion into metal NPs. But COO^– was difficult group to fit on surface of NPs. Studies have shown that AgNPs showed greater sensitivity for sensing of the different ions such as Hg²⁺ and Mn²⁺ ions at a level of 16 nM LOD. Moreover, AuNPs were found more sensitive for the detection of the Pb²⁺ and Hg²⁺ions with 16 and 53 nM LOD. Furthermore, Liu and coworkers used the biomass for the fabrication of the branched poly ethylenimine capped fluorescent carbon quantum dots (CQDs) for the selective detection of Cu²⁺ (Liu et al., 2014). Bamboo leaves extract produced branched polyethylenimine while preparation of CQDs carried out by capping the CQDs via electrostatic adsorption followed by their use for sensing of Cu ions present in river water. The high yield around 7.1 % were obtained by the carbon-QDs along with a 0.333 to 66.6 μM linear dynamic range with 115 nM of LOD. Ahmed and coworkers developed novel strategy for the recognition of the Hg²⁺ ions through calorimetry by approaching reduction through green synthesis and used polyethylene glycol poly vinylpropyl stabilized silver based NPs and studied by surface plasmon resonance in UV/Vis spectroscopy. (Ahmed et al., 2014) Among absorbance intensity of AgNPs, and concentration of Hg²⁺ ion, an inverse and linear relationship was found over the range of 10 to 1 ppm. But the 1 ppm LOD showed the absorption at 411 nm. Devnani and colleagues performed the evaluation of electrochemical sensing method about detection of metals via Au NPs prepared by the anthocyanin capped carbon paste electrode and black rice extraction produced anthocyanin (Devnani and Satsangee, 2015). Results showed that various metals (Cu²⁺, Pb²⁺ and Cd²⁺) were detected through modification of the anthocyanin capped carbon paste electrode (GNMACPE), in which a verity of surfactants and electroactive surface area was developed with 0.365 cm² intended for AuNPs/MACPE and found to be the most appropriate for metal ions sensing via voltammetry. Yu et al synthesized fluorescent carbon dots (water soluble) through eco-friendly hydrothermal approach with the Jinhua bergamot as soirce of carbon. The fabricated carbon dots possessed great stability and luminescence and act as fluorescent probe and used for detection of Hg²⁺ or Fe³⁺ ions (Fig. 2) Yu et al. (2015).

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Fig. 2

(A) Fluorescence excitation (black line) and emission (red line) spectra of C-dots. Inset shows C-dots under visible light (right) and 365 nm UV light (left). (B) The corresponding emission spectra of C-dots at different excitation wavelength from 290 to 390 nm. Reproduce with permission from (Yu et al. 2015).

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The luminescence was quenched by HAC-Na-AC for Hg²⁺ ions and Tris–HCl for the Fe³⁺ detection with low cost, quick response, specificity with wider range of 0.025 to 100 μM and 0.01 to 1000 μM along with the detection limits of 0.075 μM and 5.5 nM respectively for the Fe³⁺ and Hg²⁺ was obtained. Shi and coworkers discussed the green approach for synthesis of the oxygen-rich nitrogen-doped graphene based quantum dots (N-GQDs) for the recognition of Hg²⁺ metal ions present in the river water (Shi et al., 2015). The synthesis of N-GQDs was done via solid-phase approach by locating 3,4 dihydroxy L phenyl-alanine (L-DOPA) as citric acid. Results showed that the wavelengths about emission excitations were observed around 445 and 346 nm with 18% of the quantum yield because of the involvement of nitrogen atom to GQDs. The fabricated nitrogen-graphene-QDs fluoro-sensor was specific for the detection of Hg²⁺ due to their slaking effect of metal ions via non-radiative transfer of electrons with LOD value of 8.6 nM. Moreover, AgNPs were used to study the biopolymer Xylan which act as stabilizing and reducing agent for metal ions. Apart from this, Tollen’s reaction was applied for sensing the Hg²⁺. Studies revealed that AgNPs prepared via green methods were found to be mono-dispersed with size of >20–35 nm because of the package of Xylan (Luo et al., 2015). Although 15 other metals were present but Hg²⁺ was detected specifically with LOD of 4.6 nM. By using xylan-Ag NPs as detecting source, some real water samples were also analyzed for the detection of Hg²⁺. For the purpose of the surface modifications, Poly-dopamine has been applied widely because of its adhesive purposes of mussels. So, Annadhasan and Rajendiran used the AgNPs for the specific recognition of tap water Hg²⁺ ions. AgNPs was prepared by using N-cholyl-L-cysteine (NaCysC) which played role as reducing and capping agent at room conditions and sunlight exposure (Fig. 3) (Annadhasan and Rajendiran, 2015). According to the results, AgNPs showed average size of 7.8 nm. The fabricated AgNPs were used to detect the Hg²⁺ ions with LOD around 8 nM. It has been found in another study that successful detection of Hg²⁺ ions was done by AgNPs further stabilized with the help of a GMA (green multifunctional agent), and Burmese grape (fruit juice) in sonochemical condition (Annadhasan and Rajendiran, 2015). It has been observed via a comparative study that AgNPs with functional groups adsorbed on to the surface through active places on the biomolecules helped in changing the aggregation of nanomaterials. For Hg²⁺ ions, mono-dispersed AgNPs showed a size range between 5 and 10 nm with 47.6 μM LOD at varied pH (3.73–11.18). Similarly, detection of Ni²⁺ and Co²⁺ ions inside water, via calorimetric method was made by the N cholyl-L-valine (Na- ValC)-reduced gold nanoparticles (AuNPs) without exposure of the sunlight (Annadhasan et al., 2015). The variations in size and shape of the Na-ValC-capped AuNPs was noticed by changing the concentrations. Lower concentration gave the AuNPs average size around 40 nm and higher concentration provides 8.0 nm size. Moreover, in Na-ValC the functional groups NH–C = O and OH took part in metal ions reduction, while for the stabilization of these AuNPs hydrophilic and lipophilic moieties were involved. The coefficient of linear regression (R2) and LOD the were exist around 10 nM and 0.9870 for Ni²⁺ while 10 nM and 0.9806 for Cu²⁺ ions, respectively. Nitrogen-doped based carbon dots were prepared by using a lotus root which acted as carbon source (Gu et al., 2016). For the production of the CDs, lotus root was found to be the best choice due to presence of polysaccharides, alkaloids, glycoprotein and amino acids, act as a source for the carbon and nitrogen. One pot microwave treatment results the CDs with 5.23% nitrogen content via LR by using the fluorescent nitrogen-doping, which helped as a surface passivating agent and controlled the participation of other passivating type agents. Maximum yield of 19.0% was observed by the nitrogen-capped CDs. A great level of detection was observed by LR carbon dots for the Hg²⁺ ions with LOD of 18.7 nM. It was found after study that detection of metal ions via calorimetry is very easy and it can observe via naked eye as well (Cheon and Park, 2016). AgNPs by using mussel-inspired protein also helped to detect the Pb²⁺ and Cu²⁺ ion successfully via colorimetric method. In this case, AgNPs are synthesized and 3,4 dihydroxy L-phenyl-alanine (poly DOPA) was found involved for stabilization and reduction purpose and results showed that as-synthesized poly DOPA-AgNPs has the size 25 nm with higher dispersion. TEM images showed that accumulation of the poly DOPA-AgNPs resulted the change in color from yellow to brown and LOD of 9.4 × 10⁻⁵ and 8.1 × 10⁻⁵ M for Pb²⁺ and Cu²⁺ ions. Similarly, detection of Hg²⁺ and Fe²⁺ ions takes place via calorimetric method, for this purpose sugar bodies (dextrose and maltose) reduced AgNPs were used (May and Oluwafemi, 2016). The change in size of the sugar-reduced gelatin-capped AgNPs was observed. For maltose and dextrose-capped AgNPs, the size was found to be 11.88 and 18.17 nM, respectively. It was observed that dextrose-reduced NPs showed more sensitivity for the detection of the metal ions as compared to the maltose at higher level of metal ions. Maltose and dextrose-reduced AgNPs detected the Hg²⁺ and Fe²⁺ ions at 0.001 M and 0.1 M respectively and showed higher sensitivity for the Hg²⁺ ions up to 10⁻¹² M with a linear range of 10⁻¹ to 10⁻⁵ M. Novel method by using the egg white capped AgNPs for the sensing of Hg²⁺ ions in water have been explored. (Tirado-Guizar et al., 2017) It was found that egg white showed great activity for the preparation of the AgNps with increased specific binding positions for the Hg²⁺ ions related to the attachment of the thiol groups. AgNPs with egg white capping showed specificity for sensing of the Hg²⁺ ions in the existence of various other metals with the limit of detection of 300 nM Generally, Hg²⁺ ions detection was made by the calorimetric method via green synthesized AgNPs. AgNPs were prepared by using fruit extract such as Carica papaya in the exposure of sunlight (Firdaus et al., 2017). For the oxidation of metal ions into the zero oxidation state, ascorbic acid and phenolics are involved and bio-active compounds (water-soluble) extracted from the papaya fruit served as the bio-reductants of Ag¹⁺ to form Ag NPs and Hg²⁺ ions were detected specifically in the presence of other metals. Furthermore, more attraction was governing by AgNPs for the Hg²⁺ ions and it was observed from the colorimetric results that the Hg²⁺ ions changed the solution color from yellowish brown to the colorless. Disappearance of color takes place because of oxidation of silver metal in AgNPs to Ag¹⁺.

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Fig. 3

Schematic illustration of AuNPs formation at different time intervals under natural sunlight irradiation. Figure reprinted from Ref. Annadhasan and Rajendiran, (2015) with permission from RSC, copyright 2015.

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3.2 Heavy metal detection using chemical synthetic approaches

Mono-dispersed nanomaterials have been produced using a variety of chemical and physical techniques., e.g., phytochemical (Narchin et al., 2018) electrochemical (Singaravelan and Alwar, 2015), laser ablation (Sadrolhosseini et al., 2019), co-precipitation (Kandpal et al., 2014) and arc discharge (El-Khatib et al., 2018) techniques. These traditional synthesis techniques can simply harvest smaller and more uniform nanoparticles. Prior to high surface to volume ratio and excellent functionalities of nanomaterials, physical and chemical synthesis techniques were often employed for heavy metal recognition. Herein, Yang and colleagues fabricated Bi-NPs for selective and sensitive recognition of Cd²⁺ and Pb²⁺ ions. Reduction of Pb²⁺ and Cd²⁺ by employing Bi-NPs based modified glassy carbon electrode (GCE) provided an obvious response at 0.85 V and 0.65 V respectively. Bi-NPs found to be highly selective with detection limit of 10 g/L for Pb²⁺ and Cd²⁺ and a small particle size distribution of about 80 nm (Yang et al., 2013). Another study discovered the use of carbon xerogel Bi-NPs for recognition of lead and Cadmium ions in an electro-chemical experiment (Gich et al., 2013). The electrochemical performance of the synthesized NPs was achieved with low LOD (<1 ppb) for Pb and Cd in aqueous system. By employing the square wave anodic stripping voltammetry, Zhu and coworkers investigated the determination of Pb²⁺ and Cd²⁺ ions through Au-NPs graphene cysteine composite with glassy carbon electrode (Zhu et al., 2014). Because of the electrical conductivity and synergetic effect of Au-NPs combined with cysteine and graphene, the deposition of Cd²⁺ and Pb²⁺ ions on modified glassy carbon electrodes was critical. The LOD of Cd²⁺ (0.10 g/L) and Pb²⁺ (0.05 g/L) was found to be very low, with a linear relationship between current and Cd²⁺ and Pb²⁺ concentrations ranging from 40 to 0.50 g/L. Alternatively, for the colorimetric detection of Hg²⁺ ions, SiO@Ag core–shell NPs were used (Boken and Kumar, 2014). Chemical reduction synthesis technique was used to fabricate SiO@Ag core–shell NPs, which had a surface plasmon resonance peak at 433 nm for the Ag-NPs and 450 nm for Ag core–shell and NPs and these were found enormously sensitive to Hg²⁺ in aqueous medium with low detection limit. As synthesized Ag-NPs had shell thicknesses of 28 nm and sizes of 202 nm. Heavy metals such as Pb²⁺, Ni²⁺ and Cd²⁺ were successfully determined using a carbon paste electrode designed with Bi-NPs in an electrochemical test (Niu et al., 2015). In different water samples (ground polluted and tap water), the Bi-NPs based electrochemical sensor exhibited excellent sensitivity and selectivity for Ni²⁺, Pb²⁺, and Cd²⁺and revealed a very low detection limit of 5.47 ppb for Ni²⁺, 0.65 ppb for Pb²⁺, and 0.81 ppb for Cd²⁺ ions. Graphene and quantum dots have been broadly utilized in numerous sensing fields due to their remarkable physicochemical properties. Using an electrochemical detection method, Au-NPs-graphene quantum dots (GQDs) were developed for selective recognition of Cu²⁺ and Hg²⁺ ions (Fig. 4) (Ting et al., 2015). Because of the smaller narrow particle size 50 nm of Au-GQDS, these are found highly sensitive to Cu²⁺ and Hg²⁺ ions with ultra-low detection limits of 0.02 nM for Hg²⁺ and 0.05 nM for Cu²⁺ ions. For the detection of Pb²⁺ ions in an electrochemical system decorated glassy carbon electrodes with ferrite MnFe₂O₄ NPs in a mesoporous cluster arrangement have been used (Han et al., 2015). This electrode architecture strategy detected the Pb²⁺ with a low LOD of 0.054 nM under appropriate electrochemical parameters. The size of as-prepared MnFe₂O₄ NPs was found between 200 and 400 nm with mesoporous nature and a diameter of 8 to 12 nm. Also, Xie and co-workers described graphene/CeO₂ based hybrid material for determination of Cu²⁺, Hg²⁺, Pb²⁺ and Cd²⁺ metal ions (Xie et al., 2015). By using a hydrothermal process, cerium oxide nanoparticles with graphene hybrid caps were fabricated and integrated onto a GCE. According to the findings, as-prepared graphene/CeO₂ hybrid materials were highly selective and responsive to Cd²⁺ Hg²⁺, Cu²⁺ and Pb²⁺ ions. The reported LOD was significantly lower than the World Health Organization's (WHO) recommended values. On the other hand, Sulistiawaty et al. assessed Tween-20 and gelatin as a stabilizing agent for colorimetric detection of Hg²⁺ ions through Ag-NPs (Sulistiawaty et al., 2015). Hg²⁺ metal ions were noticed to be very selective for Tween-20 and gelatin stabilized Ag-NPs with low LODs for gelatin AgNPs (0.45 mg/L) and Tween-20 (0.13 mg/L). Tween-20 Ag-NPs had a diameter of 17.54 nm, while gelatin AgNPs had a diameter of about 9.68 nm. Iron oxide nanoparticles (Fe₂O₃ NPs) have been (Bobik et al., 2015) exploited for the adsorption of different heavy metal such as Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Cr³⁺, Pb²⁺ and Cr⁴⁺ ions. Fe₂O₃ NPs were prepared using co-precipitation technique at various temperatures and ammonia volumes resulting in different adsorption capacities for Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Cr³⁺, Pb²⁺ and Cr⁴⁺ ions. Multicomponent-NPs (MCNPs) were used for 99% elimination of As, Ag, Pb and Cu metal ions from the artificial mine tailing (Stael and Cumbal, 2016). To make MCNPs, researchers used a novel synthesis technique and employed nitrogenized water to synthesize different concentrations of Na₂SO₄, NaBH₄, and FeCl₃. Within 5 min, the As, Ag, Pb and Cu metal ions were removed more than 99.50%. Likewise, Shi and colleagues established an electrochemical valuation for the synchronized determination of Pb²⁺, Cd²⁺ and Zn²⁺ ions by using 3D graphene-framework-Bi NPs (Fig. 5) (Shi et al., 2017). The tunable morphologies of Bi-NPs were reduced electrochemically and widely distributed on the framework and providing broad active surface area for heavy metal detection. 3D graphene-framework-Bi-NPs were remarkably sensitive towards Pb²⁺, Cd²⁺ and Zn²⁺ ions metal ions (Bobik et al., 2015) and LODs were detected as 0.02 μg/L for Pb²⁺; 0.05 μg/L for Cd²⁺ and 300 μg/L for Zn²⁺ ions. Furthermore, Zhou and colleagues used L-cysteine functionalized MnFe₂O₄-glassy carbon electrode to detect different (Cu²⁺, Cd²⁺, Pb²⁺ and Hg²⁺) metal ions in a sensitive and selective manner. Under individual and simultaneous detection conditions, MnFe₂O₄/Cysglassy carbon electrodes were extremely sensitive to Pb²⁺ ions, with high sensitivity of 57.0 and 35.3 μA/μM. (Zhou et al., 2017). Cu-reduced graphene oxide (CuNPs/RGO) were synthesized through the liquid-phase reduction to electrochemically estimate different heavy metal ions (Cu²⁺, Hg²⁺, Cd²⁺ and Pb²⁺) (Li et al., 2017). CuNPs/RGO were used to detect metal ions simultaneously using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The findings established that synthesized nanocomposites were greatly sensitive to metal ions, with limit of detection values of 0.111 nM for Cu²⁺, 0.051 nM for Hg²⁺, 0.203 nM for Cd²⁺ and 0.186 nM for Pb²⁺ ions. Sarkar and his colleagues developed a colorimetricprocedurefor measuring Hg²⁺ ions in collected water sample by a model extremely efficient mercury sensor (Sarkar et al., 2017). A low-cost prototype mercury sensor based on Ag-NPs impregnated with poly vinyl alcohol (PVA-AgNPs) was used to recognize Hg²⁺ ions with LOD value of 10 ppb. Surface plasmon resonance band of silver based nanoparticles were lost due to the aggregation and amalgamation with the Hg²⁺ ions and yellowish colour of solution turns to colorless in existence of mercury ions. Glassy carbon electrode modified with use carbon NPs was used for the selective and sensitive determination of heavy metals including Cu²⁺, Pb²⁺ and Hg²⁺ ions in aqueous media (Fig. 6) (Simpson et al., 2018). Three separate reducing sources were used to make the carbon-NPs, such as (1) glycerol and silica, (2) glycerol and H₃PO₄, and (3) glycerol, H₃PO₄, and silica. The size distribution of the as-synthesized carbon-NPs was 89 nm, 66 nm, and 58 nm. Carbon-NPs based electrode was enormously sensitive with detection limits of Cu²⁺ 0.50 ppm, Hg²⁺ 31 ppm and Pb²⁺ 0.30 ppm respectively. However, Zhou and coworkers established another electrochemical assay to selectively recognize the Pb²⁺ ions with ferrite-NPs (MnFe₂O₄) incorporated with graphene oxide (Zhou et al., 2018). The GO/MnFe₂O₄ nanocomposites were uniform in size, having diameter in the range of 400 nm and porosity of 4.5 nm. GO/MnFe₂O₄ presented promising sensitivity to Pb²⁺ ions with LOD value of 0.0883 μM. Furthermore, Antunović and colleagues identified an electrochemical pathway for instantaneous detection of Pb²⁺ and Cd²⁺ metal ions using MnCo₂O₄ NPs modified bare glassy carbon electrode. MnCo₂O₄ NPs were synthesized using the citrategel combustion method and instrumental findings exhibited that NPs were mono-dispersed with a size distribution of 19 nm. LOD was found to be 8.06n mol/dm³ for Pb²⁺ ions and 7.02 nmol/dm³ for Cd²⁺ ions, respectively (Antunović et al., 2019). Cu-NPs were used to detect Ag²⁺ ions colorimetrically in the aqueous medium. Cu-NPs were found to be uniformed in size with diameter of 5.0 nm. The Cu-NPs showed good selectivity for Ag²⁺ ions with wide linear range (100–600 mol/L) (Wang et al., 2019).

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Fig. 4

Schematic illustration of Hg2+ detection. Figure reprinted from Ref. Ting et al. (2015) with permission from Elsevier, copyright 2015

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Fig. 5

Schematic illustration of Facile fabrication of a novel 3D graphene framework/Bi nanoparticle film for ultrasensitive electrochemical assays of heavy metal ions. Figure reprinted from Ref. Shi et al. (2017) with permission from Elsevier, copyright 2017.

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Fig. 6

Schematic Illustration of synthetic and detection processes of carbon nanoparticles for heavy metal ions in aqueous media. Figure reprinted from Ref. Simpson et al. (2018) with permission from Elsevier, copyright 2018.

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