In an open-access article published in the journal Molecules, researchers presented a precise model that correctly estimated the number of nanoparticles in water from the molar concentration and mass of gold nanoparticles (AuNPs). Ultraviolet-visible spectroscopy (UV-vis) and transmission electron microscopy (TEM) were used in this research to create and analyze citrate-capped gold nanoparticles.
Numerous environmental studies on nanomaterials, including their fate, toxicity, and general occurrence, depend on precisely estimating the number of nanoparticles and their size.
Environmental matrix mimics were created by adding gold nanoparticles to sediments, extracting them using leachate, and separating them from the bulk matrix using centrifugation and phase transfer separation methods. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the molar concentration of gold nanoparticles on the extracted residues.
The determination of the number of nanoparticles in segregated residues was made possible by the molar concentration, the average diameter of 27 nm, and colloidal suspension volumes of gold nanoparticles. In addition, ICP-OES was used to determine the number of nanoparticles in the samples based on the plot of the number of gold nanoparticles against the mass of gold nanoparticles.
The present study was the first application of the gravimetric approach to ICP-OES for calculating the number of nanoparticles following the phase transfer separation.
Growth of Metal Engineered Nanomaterials (ENMs) in Recent Decades
Recent decades have seen an exponential increase in the study and development of metal-engineered nanomaterials (ENMs), leading to extensive commercialization of items utilizing metal-engineered nanomaterials.
Due to the rise in their use, metal-engineered nanomaterials have been released into the environment and have potential adverse effects on human and animal health.
However, more research is needed to understand their environmental prevalence, fate, and behavior in the environment to provide suggestions to policymakers.
The ecological impact of metal-engineered nanomaterials is poorly understood due to the lack of data on their prevalence in a natural environment. The complete lack of extraction techniques and robust analytical approaches for studying metal-engineered nanomaterials contribute to the poor knowledge regarding the environmental influences of metal-engineered nanomaterials.
The concentrations of metal-engineered nanoparticles (ENPs) have been observed in various forms, such as mass. However, accurate calculation of molar concentration and, consequently, the number of nanoparticles may be the most critical attributes to studying the fate, environmental impact, toxicity, and behavior of the metal-engineered nanoparticles.
Various analytical models are created to solve these issues. However, since they depend on the makeup and dimensions of metal-engineered nanoparticles, most of these models only apply to specific types of metal-engineered nanoparticles.
Based on these techniques, other models have been created, including:
- Gravimetric measurements
- Light absorption
- Dynamic light scattering
- Laser-induced breakdown detection
- Single particle counting
- Induced plasma-linked spectrometry
- Optical sensing.
However, the most popular methodology for measuring metal-engineered nanoparticles in a natural setting is a single particle counting method based on the single particle inductively coupled mass spectrometry (SP-ICP-MS) technique. It calculates the molar concentration of metal-engineered nanoparticles in the sample using transit efficiency.
It is necessary to create universal designs that are direct to use and particular to the sizes and shapes of metal-engineered nanoparticles since the rate at which they are made and utilized is accelerating.
A bottom-up approach that started with producing metal-engineered nanoparticles, characterization, spiking, extraction, separation, and quantification was proposed to meet the requirement for a precise way to quantify the number of nanoparticles in the sample.
Finally, the gravimetric approach was adapted to demonstrate the relationship between the mass of gold nanoparticles obtained from molar concentration and the number of gold nanoparticles. This relationship model was predicted to apply to various types of nanoparticles that could be distinguished and studied by ICP-OES.
Conducting the Experimental Investigations
A wet chemical technique, a bottom-up strategy, was used to create metal-engineered nanoparticles. The UV-vis spectrometric characterization of the synthesized metal-engineered nanoparticles was performed. This spectroscopy technique helped investigate the optical characteristics of metal-engineered nanoparticles.
High-resolution transmission electron microscopy (HRTEM) helped determine the gold nanoparticles' sizes and morphologies. For HRTEM characterization, fresh nanoparticles were produced and sonicated before analysis. Such a procedure was necessary to prevent precipitation which can alter the morphology of the nanoparticles and make it difficult to determine the impact of each reducing agent on size and shape.
Leachate, which simulated the appropriate environmental processes to extract sediments into supernatants, was employed to extract nanomaterials from bulk matrices. The total molar concentration of gold that created specific nanomaterials in the solution was then assessed using the centrifugation and phase transfer procedures.
The ability to extract nanomaterials from bulk matrices using the phase transfer segregation technique was established. However, the presence of nanomaterials in the intermediary phases caused significant phase transfer problems. 20 mL of diluted supernatant from 0.5 g of sediment was spiked with 100 L of gold nanoparticles and separated into 6 mL of toluene to test the phase transfer procedure.
ICP-OES analysis of the aqueous, intermediate, and organic phases ascertained the portion of the separated sample that contained gold nanoparticles. As was previously mentioned, toluene and the intermediate phase had significantly more gold nanoparticles than the aqueous phase. Therefore, the toluene and intermediate phase residues were mixed and subjected to ICP-OES analysis.
The traces of gold discovered in the aqueous phase after ICP-OES analysis were ascribed to the unreacted gold during the reductions. Finally, the phase transfer separation method could accurately and effectively separate gold nanoparticles from dissolved gold metal ions because the gold salt was not entirely converted to gold nanoparticles.
A centrifugation separation technique assessed the segregation of gold nanoparticles from supernatants. In the centrifugation procedure, 1 g and 0.5 g of sediments were spiked in triplicate with various amounts of gold nanoparticles, extracted and separated, and subsequently digested and analyzed by ICP-OES. After the centrifugation, the gold nanoparticle residues were digested and subjected to ICP-OES analysis.
The sediment-leachate systems were placed into 50 mL centrifugation tubes. They were then exposed to centrifugation for 20 minutes at 2000 rpm to segregate the supernatant from the sediments during the centrifugation separation of gold nanoparticles. Since the system was spiked with spherical gold nanoparticles, the centrifugation extraction method extracted spherical nanoparticles into the supernatant phase since it assumed that the suspended nanomaterials were spherical and behaved according to Stokes' law. Finally, the supernatants were decanted into a 50 mL beaker after the completion of centrifugation.
For the phase transfer segregation of gold nanoparticles, 0.3354 g of octadecylamine (OCTDA) was measured and diluted in 100 mL of toluene to create a 0.01 molar concentration solution. Additionally, the effectiveness of the phase transfer technique was confirmed by digesting the known molar concentration of gold nanoparticles without extraction by OCTDA toluene solution.
Findings and Significance of the Study
The molar concentration and the number of nanoparticles are essential for quantitative analysis. Thus, the findings of this work would help environmental scientists investigate nanoparticle occurrence with useful modeling information.
Leachate, centrifugation, and phase transfer procedures were used to separate gold nanoparticles from the bulk matrix. The authors measured the size and molar concentration using HRTEM and ICP-OES, respectively.
Molar concentration and size parameters were then fitted to the modified gravimetric formulae to determine the number of nanoparticles in a sample. The findings revealed a relationship between the mass of AuNPs and the number of nanoparticles in a sample. This relationship could estimate the number of nanoparticles in the environment.
The authors believe that the proposed approach would make it easier to use HRTEM and ICP-OES, two commonly used laboratory techniques, to analyze the number of nanoparticles in the environment.
Hendricks, N., Olatunji, O., Gumbi, B. (2022). A Simple Model to Estimate the Number of Metal Engineered Nanoparticles in Samples Using Inductively Coupled Plasma Optical Emission Spectrometry. Molecules, 27(18), 5810. https://www.mdpi.com/1420-3049/27/18/5810/htm