Short Communication
Silver and gold nanoparticles as chemical probes of the presence of heavy metal ions

https://doi.org/10.1016/j.molliq.2020.112559Get rights and content

Highlights

  • A simple and inexpensive method of nanosilver and nanogold particles production in the presence of lignosulfonates

  • Detailed physicochemical and morphological characterization of silver and gold nanoparticles

  • Silver and gold colloids are good indicators of the presence of heavy metal ions.

Abstract

Technical sodium lignosulfonate, a natural polymer, was used as reducing and stabilizing agent for a simple and inexpensive synthesis of silver and gold nanoparticles (AgNPs and AuNPs) in aqueous solution at room temperature. Further, AgNPs and AuNPs were used for spectrophotometric detection of selected heavy metals ions (Cd2+, Cu2+, Co2+, Pb2+ and Ni2+) in model aqueous solutions. The obtained AgNPs and AuNPs were characterized by UV–vis spectrophotometry, size distribution (DLS) and transmission electron microscopy (TEM). The results clearly indicate that received silver and gold colloids can be used as chemical probes of the presence of heavy metal ions if the interference of other species is excluded.

Introduction

Transition metal nanostructures can be obtained by many methods, however biological methods are the most environmentally friendly, fast and inexpensive. Some bacteria, fungi or plants can be used for this purpose.

It is well known that metals are toxic to a significant number of various microorganisms. On the other hand, in nature, there are some bacteria resistant to certain metals, i.e. silver and gold. They are not susceptible to the effect of nanosilver and are able to reduce it, including bacteria Pseudomonas stutzeri AG 259 or Proteus mirabilis PTCC 1710 [1].

Due to the ease of bioaccumulation of metals and formation of bonds, fungi such as Verticillium and Fusarium oxysporum were also used in the synthesis of AgNPs. Depending on the type of fungi used, the synthesis process can take place relatively quickly - even within a few minutes. In addition, it has been proven that Aspergillus flavus fungi are an effective biological tool for the extracellular biosynthesis of stable AgNPs, which in turn can be used as an additive to many groups of antibiotics (e.g. gentamycin, vancomycin, and ciprofloxacin), which are not yet ineffective against certain bacteria [2,3].

However, researchers take the greatest interest in the method of obtaining nanosilver with the use of plant extracts, i.e. extracts of pear, papaya, coffee, tea or chrysanthemum [[4], [5], [6]]. It is a fairly economical and not very complicated method, whereby plant extracts contain substances that simultaneously can reduce and stabilize the produced nanoparticles. These substances include, among others, citric acid, polyphenols or flavonoids. The use of tea leaf extract allowed to obtain very stable nanostructures of silver gold specimens with small diameters – 30 and 10 nm, respectively, which were later used as antibacterial pigments added to cotton fabrics [5].

Natural polymers, such as starch, dextrin or lignosulfonates, a by-product formed in the wood processing can also act as effective reducers and stabilizers of AgNPs and AuNPs [[7], [8], [9]]. During the production of paper using the sulfite method, lignosulfonic acids are formed, which react with salts or bases, forming calcium, sodium, magnesium and ammonium salts. The resulting lignosulfonates are anionic polyelectrolytes, derivatives of lignin, containing a large number of sulfonic groups. The elemental sulfur content in lignosulfonates is higher than in other technical lignins and amounts to 4–5% w/w [10]. During this process, lignin is sulfonated, degraded and the reaction products are water-soluble. Lignosulfonates are produced worldwide in a relatively large amount, approx. 1 million tonnes of dry matter per year. The characteristic structural and chemical properties of lignosulphonates are a relatively high molecular weight (15,000–50,000 g·mol-1), a high degree of polydispersity, solubility in water and a relatively high ash content [11]. In aqueous solutions, due to the presence of hydrophobic chains and numerous hydrophilic functional groups in their structure, they have surface-active properties and have a strong tendency to aggregate [12]. Lignosulfonates contain in their structure a large number of various functional groups such as carboxyl, phenol, sulfone and other sulfur-containing groups [[13], [14], [15]]. This implies numerous potential industrial applications [16,17], however, technical products take the form of the salts of cations present in spent pulping lye in the extraction process. These cations affect the subsequent reactivity of lignosulfonates. The presence of an ammonium cation generates the most reactive lignosulfonates, while lignosulfonates with a calcium cation present the weakest reactivity. The reactivity of sodium and magnesium lignosulfonates is of secondary importance [[18], [19], [20]].

Due to the toxic effects of heavy metals on individual environmental components and their bioaccumulation in the food chains of humans and animals, these metals pose a significant threat to the environment. In recent years, a number of studies have been conducted on the sources and emissions of heavy metals into water, soil, atmosphere and the way they enter the environment. Heavy metals occur in the form of sprayed metal or oxide, while in ground, surface waters, and soil they take an ionic form. Human activities have an increasing impact on the circulation of these elements in nature. This is due to technological development, extraction and processing of natural resources, as well as extensive consumption of manufactured products.

As a result, huge amount of waste is formed, from which dusts and gases are emitted to the atmosphere, sewage is discharged into surface waters, and waste is collected in settlers or landfills. In waters, plants and soil surface layers, heavy metals undergo various chemical and biochemical transformations. This has a great environmental impact. The consequence of the above phenomena is an increase in the concentration of elements and chemical compounds in the hydrosphere, atmosphere and surface layers of the lithosphere. Despite the research carried out on the utilization and neutralization of soils and sediments contaminated with heavy metals and their removal from water and sewage, they still will be considered as problematic to the environment for many years to come.

The aim of the conducted research was to develop nanostructured silver and gold with the participation of sodium lignosulfonate, and then their further use for spectrophotometric detection of selected heavy metals ions (Cd2+, Cu2+, Co2+, Pb2+ and Ni2+) in model aqueous solutions. This was possible due to the fact that AgNPs and AuNPs nanostructures belong to the group of nanochromophores, and their characteristic feature is an absorption band at wavelength (λmax) 400–450 nm for nanosilver and about 530 nm for nanogold [21,22]. Due to the addition of heavy metal ions, a change in the colour of the samples is observed. This is accompanied by a shift in the UV spectrum. The second, extensive band appears, which indicates the formation of Ag(Au)NPs-heavy metals ions aggregates.

Section snippets

Materials

Sodium salt of lignosulfonic acid was used as a reducing and stabilizing agent in the synthesis of AgNPs and AuNPs. Description of sodium salt of lignosulfonic acid action was presented in detail in the paper Milczarek et al. [23]. The sample was provided by Borregaard LignoTech (Sarpsborg, Norway). This lignosulfonate is a water-soluble lignin derivative, subject to the ultrafiltration process, with under 0.1% sugar content.

An ammoniacal solution of silver(I) nitrate was used as the source of

Particle size distribution and TEM images

The first stage of the study involved an analysis of particle size distribution in the obtained silver and gold nanoparticles. A particle size seems to be crucial for the stability of these AgNPs or AuNPs solutions. It is well established that larger particles tend to sediment after a short time, while smaller particles may remain stable for months.

Table 1 summarizes the ranges of particle diameters observed in the samples, the dominant particle diameter in each, along with its percentage share

Conclusions

To summarize the effect of selected metal ions on the absorption spectra of AgNPs and AuNPs, in the presence of these metal ions the intensity of the original plasmonic band is reduced along with occurrence of a new, broad absorption band. These results clearly indicate that silver and gold colloids can be used as chemical probes of the presence of heavy metal ions if the interference of other species is excluded.

CRediT authorship contribution statement

Anna Modrzejewska-Sikorska: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Resources; Writing - original draft, review & editing. Emilia Konował: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Resources; Writing - original draft, review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Polish Ministry of Science and Higher Education (Grant No. 0911/SBAD-0398).

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