2.2. Nanorelease Study
The mechanical stress test of this study aimed to stress the surface of the nanocomposites and to evaluate whether release of nanomaterials (nanomaterials) under stress conditions was possible. The setup of the stress test must therefore take influence on the surface of the nanocomposite whilst at the same time released nanomaterials must be picked up to be detected by following analytical techniques. The idea was to use an abrasive substance that scrubs the nanocomposites surface. In a further step the abrasive substance and the abrasion itself were collected completely and analysed by suitable techniques on the presence of nanomaterials. The basic setup of the nanorelease test is schematically displayed and described more in detail in part A of this series of publications.
2.2.1. Basic Material Stressing—Abrasion Test
Nanocomposites were clamped in glass Petri dishes and loaded with 1.0 g of an abrasive substance (i.e., salt or quartz sand). Depending on the chemical nature of the nanomaterial and the subsequent analytical technique used either quartz sand (Büchi Labortechnik GmbH, Germany) or salt (sodium chloride p.A., Sigma Aldrich Chemie GmbH, Germany) were used as a dry abrasive simulant. The cells were composed of a smaller petri dish with an inner diameter of 90 mm and a larger petri dish with an inner diameter of 95 mm. In a first step, the smaller cell was loaded with the dry food simulant and the nanocomposite was loosely placed on the dish. In a second step the larger cell was put over the smaller cell with the nanocomposite film in between. This way the nanocomposite was clamped in the cells with an even surface (Figure 4
). Furthermore, the nanocomposite itself acted as a sealant between the cells to prevent loss of dry simulant and abrasion. By turning the cells upside down (small cell on top) the dry food simulant was then brought in contact with the nanocomposite. The surface area of the nanocomposite that was in contact with the dry food simulant was given by the inner diameter of the smaller cell (90 mm), which was 0.64 dm2
The abrasion test was started by placing the cells on an orbital shaker (GFL 3017, Gesellschaft für Labortechnik, Germany). The rotating movement of the cells caused the abrasive substance to homogeneously scrub over the nanocomposite surface. The intensity of the stress test could be varied by duration and the frequency of the shaking as well as by the amount of abrasive substance.
For the abrasion test all cells were equipped with the nanocomposites as described above. In practice it turned out that best results can be achieved when the dry food simulants are stored in a desiccator before testing, to prevent adhesion on the nanocomposites surface caused by humidity.
The speed of the laboratory shaker was set to 275 rpm. This speed caused a flat distribution of the dry simulant on the nanocomposite, whereby the surface was always nearly completely covered. Also, at 275 rpm the simulant showed a homogeneous rotating movement over the nanocomposites’ surface, meaning that all parts of the nanocomposite were abraded evenly. For the surface abrasion test all samples were abraded with the dry food simulant for 60 min (without prestress conditions) or 30 min (with prestress conditions applied).
2.2.2. Additional Material Stressing—“Prestress Conditions”
As described in part A of our other article prestress conditions can be applied to nanocomposites in advance to gather additional stress conditions that might appear in practice. In real life, the food contact material could be exposed to other mechanical impacts, thermal cycles or aggressive chemicals which could weaken the polymer and thus facilitate the release of nanomaterials after abrasion. Such potential stress situations were simulated in this study as follows.
Stripes of 50 cm of the nanocomposites were loosely folded, placed in closed foil pans, and stored in a temperature controlled oven for 24 h at 100 °C. After cooling down to room temperature all samples were placed in abrasion cells and subjected to the abrasion test for 30 min.
Stripes of 50 cm of the nanocomposites were loosely folded, placed in closed foil pans and stored in a temperature controlled freezer for 24 h at −50 °C. After warming up in a desiccator (to prevent condensate formation) all samples were placed in abrasion cells and subjected to the abrasion test for 30 min.
Solvent based stress—swelling:
Stripes of 20 cm of the nanocomposites were fan-folded, placed in stainless steel cells and filled with 100 mL isooctane, whereby the test films were completely immersed in the simulant. The cells were closed and sealed with a PTFE ring and stored in a temperature controlled oven for 24 h at 40 °C. Prior to mechanical stressing all samples was stored for 12 h under a laboratory hood to evaporate any remaining solvent. All samples were placed in abrasion cells and subjected to the abrasion test for 30 min.
Cut-outs of 200 × 140 mm of the nanocomposites were clamped in a universal tensile strength testing apparatus equipped with a traverse path sensor. The nanocomposites were stretched lengthwise for 100 mm. After stretching the nanocomposites showed slight deformation. LDPE plates were slightly longer (approx. 10 mm) and showed a slightly rippled surface. From all test samples circular cut-outs from the center of the stressed films were prepared and placed in the abrasion cells for abrasion testing. Abrasion testing was performed for 30 min.
2.3. Analytical Set-Up for the Detection of Released Nanomaterials in the Abrasion
Silver is an element that can be measured at very low detection limits by inductively-coupled plasma mass spectrometry (ICP-MS). For the detection of silver in samples taken from the abrasion test the instrument was calibrated using Merck VI ICP Multielement standard solution (Merck KGaA, Darmstadt, Germany). From this stock solution a serial dilution with 0.0 (3% nitric acid blank),0.5, 1.0, 1.5, 2.0, 2.5, 5.0, 10.0, and 25.0 ng/mL silver in 3% nitric acid was prepared. The standard with the lowest concentration of silver (0.5 ng/mL) could still be distinguished from the blank and standards with higher silver concentrations. Thus, 0.5 ng/mL was set as the limit of detection of the device for silver. Quartz sand was used as an abrasive substance for silver nanocomposites because no interferences between solubilized silver particles and other ions can be expected. Stability tests were performed to evaluate whether loss of sample might adhere due to adsorption of silver on the surface of the sand particles. For this 1.0 g quartz sand was given to 20 mL of 10.0 ng/mL silver in 3% nitric acid solution and shaken for 5 min. The supernatant was analyzed by ICP-MS and compared to a 10.0 ng/mL silver standard without sand. With a stability rate of 102.7% silver could be fully recovered.
Proof of TiN NPs in the abrasion was intended to be performed by ICP-MS measurements of Ti. For this the ICP-MS was calibrated externally using a Ti standard solution (Merck KGaA, Darmstadt, Germany, Matrix: 3% nitric acid (HNO3)). From this stock solution a serial dilution with 0.0 (3% nitric acid blank), 1.0, 5.0, 10.0, 15.0, and 20.0 ng/mL titanium in 3% nitric acid was prepared and used for calibration of the ICP-MS. The standard with the lowest concentration of titanium (1.0 ng/mL) could still be distinguished from the blank and standards with higher titanium concentrations. Thus, 1.0 ng/mL was set as the limit of detection of the device for titanium. NaCl was tested on its suitability as a dry abrasive simulant in the abrasion test. For this, NaCl was tested on impacts of the Ti recovery rate. The Ti standards (5 ng/mL) in a NaCl/HNO3 matrix (1.0 g NaCl per 20 mL HNO3 (3%)) were compared to a 5 ng/mL Ti standard in 3% HNO3 matrix without NaCl. It turned out that 79.6% (average value from samples prepared in triplicate) of Ti could be recovered.
In a previous study [3
] it was demonstrated that AF4/MALLS was suitable for the characterization and quantification of laponite particles dispersed in an aqueous surfactant solution (2000 mg/L NovaChem). However, AF4/MALLS showed limitations caused by superimpositions in the fractogram when the sample matrix became too complex. With the empiric formula Na0.7
laponite is mainly composed of ubiquitous elements that do not allow sensitive detection via element-specific ICP-MS measurements, due to an already high background of laponite relevant elements. However, a combination of both techniques (i.e., AF4 with MALLS and ICP-MS as detection system) would allow simultaneous screening for particulate structures with an elemental composition typical for laponite.
For calibration of the AF4/MALLS system laponite reference dispersions in NovaChem surfactant solution at concentrations 0.0 (NovaChem blank), 250, 500, 1000, 2000, and 2500 ng/mL were prepared. The standard with the lowest concentration of laponite (250.0 ng/mL) could still be distinguished from the blank and standards with higher laponite concentrations. Thus, 250.0 ng/mL was set as the limit of detection of the device for laponite. To determine a suitable dry food simulant test with quartz sand and NaCl in the surfactant solution were performed. One gram of dry simulant was weighed out into measuring vials and filled with 20 mL surfactant solution (2000 mg/L NovaChem) used as dispersant for laponite. After the samples were shaken for 5 min the salt was completely dissolved whilst the sand caused a slight opalescent supernatant. In fact, injections of these samples showed that supernatants of samples prepared with sand caused superimpositions in the AF4 fractogram, whilst samples prepared with sand did not. Thus, NaCl was further tested on its ability to be used as a dry food simulant for laponite particles. Laponite dispersions were prepared for recovery experiments whereby a 2500 ng/mL Laponite dispersion with an additional 1.0 g NaCl per 20 mL surfactant solution was measured against a 2500 ng/mL Laponite dispersion without salt in the surfactant solution. AF4/MALLS measurements demonstrated that approx. 65.3% of the laponite could be recovered as particles. The loss of signal intensity can be explained by the high ionic strength of the dispersion due to the high content of salt, which affects the dispersion stability of clay particles and might cause sedimentation of sample.
In regard to the later measurements of samples prepared from the abrasion test superimpositions might disturb an unambiguous detection of laponite in the AF4/MALLS fractogram. Thus, AF4/MALLS/ICP-MS measurements were performed to see whether via online detection of the laponite-specific element magnesium (Mg) a better differentiation can be achieved. Injections of a 2 mg/L laponite dispersion caused a signal at the relevant elution times in the fractogram recorded with the MALLS detector (black curve, Figure 5
) but also in the fractogram recorded via ICP-MS (red curve for Mg, Figure 5
) whilst samples prepared from the dry simulant itself (i.e., salt in surfactant solution) did not cause a signal. These measurements showed that in case of release of laponite a signal in both detection modes, AF4/MALLS and AF4/ICP-MS will be recorded.
After completion of the abrasion test the sand or salt and abrasion was carefully transferred into 50 mL centrifugal vials. Abraded Nanocomposites were rinsed with the respective solution (3% HNO3 in case of Ag-NPs or TiN nanocomposites or surfactant solution in case of laponite nanocomposites) whereby the rinsing fluid was collected into the centrifugal vials together with the abrasion and the sand or salt. All vials were the filled with the respective solution to the 20 mL mark and shaken on a laboratory shaker for 15 min to detach or disperse possibly released nanomaterials.
AF4 measurements were carried out with an “AF2000 MT Series mid temperature” (Postnova Analytics, Landsberg, Germany). The system was equipped with a 500 µm channel and a polyethersulfone membrane (cut-off: 10 kDa). Characterization of particles was performed online using a 21-angle-MALLS detector “PN3621” (Postnova Analytics, Landsberg, Germany). ICP-MS measurements were performed using a 7700 series ICP-MS system (Agilent Technologies, Santa Clara, CA, USA). The ICP-MS was equipped with an ASX-520 autosampler (Agilent Technologies, Santa Clara, CA, USA) and a micromist nebulizer (Agilent Technologies, Santa Clara, CA, USA). The system operated at 1550 W plasma power, 0.3 rps peristaltic pump speed, and 15 l/min carrier gas flow rate (argon).