Transformation of zinc oxide nanoparticles in synthetic lung fluids.

Surface coatings, including paints, stains, and sealants, have recently become a focus of "nano-enabled" consumer product engineering. Specifically, zinc oxide (ZnO) nanoparticles (NPs) have been introduced to surface coatings to increase UV resistance. As more "nano-enabled" products are made available for purchase, questions arise regarding their long-term environmental and human health effects. This study tracked the transformation of NP additives commonly added to consumer paints and stains using ZnO NPs as a model system. During product application and use, there is a risk of inhalation of aerosolized ZnO NPs. To investigate the potential chemical interactions and transformations that would occur after inhalation, ZnO NPs were incubated in two synthetic lung fluids (SLFs). Initial studies utilized ZnO NPs dispersed in Milli-Q water (control), or a commercially available deck stain. Additionally, two commercially available products advertising the inclusion of ZnO NP additives were evaluated. Subsamples were taken throughout incubation and analyzed via atomic absorption spectroscopy to determine both the total (including particulate) zinc concentration and dissolved (non-particulate) zinc concentration. Results indicate that the vast majority of ZnO transformation takes place within the first 24 h of incubation and is primarily driven by SLF pH and composition complexity. Significant dissolution of ZnO NPs was observed when incubated in Gamble's solution (between 25 and 68% depending on the matrix. Additionally, all ZnO solutions saw near immediate dissolution (~ 98-100%) within 3 h of incubation in artificial lysosomal fluid. Results illustrate potential for NPs in consumer products to undergo significant transformation during use and exposure over short time periods. Supplementary Information: The online version contains supplementary material available at 10.1007/s11051-022-05527-y.

Introduction

The use of engineered nanomaterials (ENMs) in consumer products has gained popularity over the last two decades due to their advanced properties including increased strength, antimicrobial activity, and UV resistance [1-4]. Specifically, metal and metal oxide nanoparticles (NPs) have found extensive commercial and consumer application in surface coatings, including paints, stains, lacquers, and sealants [5-7].

Zinc oxide nanoparticles (ZnO NPs) are one prominent species becoming common in consumer products. ZnO is a convenient, colorless additive that is commonly used in antifouling paints [8]. In addition to its relative transparency, ZnO NPs in particular have shown resistance to microbial attack, dirt accumulation, UV damage, and general aging when applied as a surface coating [9]. While the chemically active nature of these NPs provides advantageous characteristics for consumer products, it is concerning with regard to environmental and human exposure.

Previous research regarding the environmental issues with ENMs has focused on their release into the surrounding environment and potential for transformation and degradation [10-16]. Research regarding the passive release of ENMs is dependent on the NP species, including the NP’s size, shape, chemical makeup, and ionization [11, 15–17]. However, environmental release represents only one potential exposure pathway for ENMs in consumer products. Previously, research regarding consumer health and ENMs has focused on physical release and dermal exposure [11, 15, 17–20]. A wiping method has been used to track the release of copper particles, ZnO, and CeO2 NPs from micronized copper azole (MCA) pressure-treated lumber to assess dermal release concerns [17, 19, 21, 22]. Additionally, Taber abrasion and sanding have also been used to model dermal release [23, 24]. Results indicate that the matrix in which the NP is applied, as well as the product weathering, affects the extent of NP release [23, 24]. These prior research studies have demonstrated that release is driven by the matrix, as opposed to the NP identity [25].

Industrial application of surface coatings onto outdoor facades and materials such as lumber is often completed using high-pressure application techniques, as opposed to brush or roller application, for improved efficiency [26]. This process involves forcing the product through a high-pressure application tip to form a fine aerosol [26]. The generation of the aerosolized NPs will increase the potential for inhalation by the applicant [27]. The concentration of NPs present in the aerosolized product can enable exposure to high doses of NPs and NP additives, primarily via inhalation [28]. Consumer safety practices, including poor protective equipment during application, increases the likelihood of product inhalation [28, 29]. Consumers exposed to NPs are not exposed to pristine materials. Nanoparticles will undergo physical and chemical alternation based on the matrix in which they are dispersed, and exposure will occur in the presence of other chemical present in paints, stains, and sealants.

Currently, the potential negative health impact associated with inhalation of ZnO NPs in consumer paints, stains, and sealants is difficult to assess given the myriad of potential biological interactions. Previous studies on the impacts of ZnO NP inhalation have focused on pristine particles, and most of these studies have reported negative biological responses to test particles driven by dissolution of the ZnO into more mobile species [30-34]. However, other studies have proposed a more complex particle mediated mode of toxicity [35, 36]. Alternatively, zinc-based therapies to treat a variety of respiratory issues including asthma[37], cystic fibrosis[38], and even COVID-19 infection[39] have been proposed. The impact of ZnO NP inhalation in the presence of other chemicals in a complex matrix has not been previously investigated.

This study tracks the physicochemical transformation of NPs in consumer surface coatings after exposure via inhalation. ZnO NPs, commonly found in consumer-available paints, stains, and sealants, were used as a model to simulate consumer exposure to particles during application. Specifically, the goal of this study was to track the dissolution of ZnO NPs using multiple matrices in which ZnO NPs were dispersed and SFL incubation fluids and determine the primary solution characteristics driving NP dissolution. Critically, particular attention has been given to understanding changes in chemical speciation of the ZnO NPs during incubation using X-ray absorption spectroscopy (XAFS). The presented work illustrates how NPs evolve during interactions with both the product matrix and the SLFs. The collected XAFS data should be of great interest to toxicologist in helping to determine the triggering effect of ZnO toxicity in previously published work.

Materials and methods

Nanoparticle characterization

Zinc oxide NPs were obtained from BYK (Germany) as liquid dispersions (40 wt%) listed as 20 nm in size. The product is sold as an additive for water-based surface coatings, such as stains, for UV protection. This product has been characterized in detail in our previous work [21, 22, 40]. Briefly, total metal concentration was determined via ICP-OES (Agilent 5110). Particle size and morphology was further determined with scanning transmission electron Microscopy (STEM JEOL JEM-2010F equipped with a high-angle annular dark field detector (HAAFD, Peabody, MA), and field emission scanning electron microscopy (FESEM JEOL JSM-7600F Tokyo, Japan) [21, 22]. Particle speciation was determined using X-ray adsorption fine structure (XAFS) spectroscopy [21, 22, 40]. Additional images and information regarding NP characterization can be found in the supporting information (Table S1, Figure S1-S2).

Stain characterization

Three different wood stain solutions were used in the current study all manufactured/produced by the same company. The first was a “light walnut” wood stain without ZnO NP additives labeled control stain. The two remaining products were also “light walnut” in color and advertised the inclusion of ZnO NP additives for UV protection. These products are labeled Product A and Product B. All stain products were characterized for total metals concentration using ICP-OES, and major organic components via FT-IR similar to our previous publications [21, 22]. Products A and B were also subject to STEM imaging using a Thermo Fisher Talos F200X in both TEM and HAADF mode. Finally, speciation of NP additives was assessed using XAFS outlined below.

Synthetic lung fluid selection and preparation

Many SLFs have been used in the literature to infer the biological implications of particle inhalation. In this study two SLFs were selected, Gamble’s solution (GS, pH 7.4), and artificial lysosomal fluid (ALF, pH 4.5). These fluids were selected based on their widespread use in the literature to understand the dynamics of particulate dissolution and bioaccessibility of metal particulates [30, 32, 41–46]. GS was specifically chosen as a proxy for the interstitial fluid within the lung, specifically the surfactant fluids release by alveolar cells [43]. Additionally, ALF was selected as a proxy for the intercellular environments, particularly uptake by lysosomes [42, 43]. Critically, ALF has proven to be a better proxy for intercellular environments relative to less complex buffers at pH 4.5 [41]. Details of SLF composition can be found in Table S2. GS and ALF were prepared at the beginning of each two week sampling period following the method described by Guney et al. [46]. These incubation fluids were adjusted to their respective pH values with 1 M NaOH or 1 M HCl when necessary.

Sample handling and preparation

ZnO NP suspensions were prepared in a series of solutions. Initially stock solutions of ZnO NPs were dispersed in Milli-Q water (control) or the control stain to a final concentration of 4 wt% as directed by the manufacturer, resulting in a Zn concentration of approximately 20 g/L [22]. To model the transformation of these ZnO NP additives, solutions were further incubated in 40 mL of either Milli-Q water (control), GS(GS, pH 7.4), or ALF (ALF, pH 4.5) in a 1:100 ratio by volume [46]. Milli-Q water was adjusted to pH 7.0 using 0.1 M HCl and 0.1 M NaOH before incubation. The two commercially available nano-enhanced (ZnO NP) deck stains, product A and product B, were incubated in Milli-Q water and both SLFs. Each treatment was run in triplicate. Samples were incubated at 37 °C to simulate physiological temperature while undergoing constant end over end rotation using a Benchmark Scientific Rototherm Plus.

Sampling procedure and analysis

Subsamples from incubation solutions were collected after 3, 6, 24, and 72 h, as well as after 1 and 2 weeks to assess transformation in both the short and long term. At each time point, incubations solutions were centrifuged at 4000 rpm (3200 g) for 15 min to separate heavier aggregates from suspended NPs. This centrifugation allowed for sampling of only the suspended or dissolved material of interest for further study. A total of 10 mL of supernatant were removed from each incubation solution and fresh incubation fluid added to maintain constant volume. Any material collected at the bottom of the incubation samples was easily resuspended using a benchtop vortex mixer for 20 s. Collected supernatant was further segmented to distinguish between particulate and non-particulate zinc fractions. A portion of the subsamples was run through a 10 kDa (~ 3 nm) filter unit (Amicon), centrifuged at 6236 rpm (4500 g) for 20 min to determine the concentration of ionic zinc, while the remaining portion of the sample determined total zinc concentration. Each subsample was acidified to 2% HNO3, diluted when necessary, and zinc concentration determined by atomic absorption spectroscopy using a Perkin Elmer Analyst 3100.

X-ray absorption spectroscopy

Zinc speciation analysis was completed at Materials Research Collaborative Access Team (MRCAT) beamline 10-BM at the Advanced Photon Source, Argonne National Laboratory (Lemont, IL). To determine changes to zinc speciation after incubation with SLFs, an incubation was run as described above, for 3 h. Following this, the samples were centrifuged (6236 rpm for 15 min), and the incubation fluid was removed, discarding easily suspended zinc in solution. The remaining solid was resuspended in the corresponding incubation fluid (Milli-Q water, GS, or ALF) to collect aggregated materials. The resulting liquid was filtered using 0.45-µm nylon filter (Whatman) vacuum filtration. The filters were dried, cut, and sealed within Kapton tape before analysis. Spectra were obtained at the Zn k-edge energy of 9659 eV, and scans were collected from − 200 to 1000 eV above the k-edge. Data collection was completed in fluorescence mode. The resulting XAFS spectra (2–3 scans) were merged, calibrated, and normalized using the software Athena [47]. Athena linear least-squares combination fitting (LCF) of the near edge (XANES) region was applied on the sample spectra, as well as on multiple reference compounds, from -20 eV below the edge to 60 eV above the edge. Reference spectra as well as examples of LCF results are presented in the Supporting Information (Figure S3-S4).

Results and discussion

ZnO particle characterization

The ZnO NPs obtained from BYK and used in this study were extensively characterized previously [21, 22, 40]. Although the particles are listed as 20 nm in size, a wide distribution of particle shape and size is evident in Figure S1. Regardless of morphology, the particles were verified as crystalline ZnO using both XAFS (Figure S2) and X-ray photoelectron spectroscopy (not shown).

A summary of the major metals and speciation of zinc present for the control stain, product A, and product B can be found in Table 1. The concentrations of Fe and Cu are similar for each product, likely helping produce the light walnut color; however, there are clear differences in the zinc concentration between the products A and B. Concentration of Zn in the control stain was substantial but much lower in concentration compared to products A and B. The increase in zinc concentration for the commercial products is expected, as they are advertised to contain ZnO additives for increased performance. However, the difference in zinc concentration between products A and B was not marketed by the manufacturer (Table 1). To characterize the general makeup of the organic material, present in the stains, a subsample of each product was air dried and analyzed via FT-IR. Major peaks for all three products align indicating similarity on composition. (Figure S6).

Table 1

Characterization summary for stain products used in this study. Major metals concentrations were determined via microwave-assisted digestion followed by analysis via ICP-OES. XAFS analysis represents results from linear combination fitting using the first derivate of normalized spectra from −20 eV below the edge to 60 eV above

Major Metals (mg/L)
	Control Stain	Product A	Product B
Fe	71 ± 16	76 ± 13	143 ± 70
Cu	26 ± 6	14 ± 0.7	12 ± 0.5
Zn	341 ± 62	1,825 ± 17	967 ± 37
XAFS Analysis (Relative %)
	Control Stain	Product A	Product B
ZnO	-	26	25
Zn-Organic	65	67	64
Ionic	7	7	11
Zn-Fe2O3	28	-	-

STEM micrographs were collected to determine differences in particle size and morphology and EDS spectra and maps were collected to determine chemical composition and colocalization of elements in the micrographs. A representative image of particles in product A is shown in Fig. 1, highlighting spherical ZnO particles of approximately 20-nm diameter, along with iron oxyhydroxides NP rods. Similar iron oxyhydroxides rods (shape and size) were also identified in product B as shown in Fig. 2. The size and morphology of the ZnO NP present in the stain differed substantially from product A. ZnO particles present in Product B are significantly larger on the order of 50–100 nm in length and are rectangular in shape as opposed to the spheres that were identified in product A. As both products come from the same manufacturer, the inclusion of different size ZnO particles appears intentional. The same iron oxyhydroxides rods were also identified in the Control Stain, (Figure S7) again indicating the similarity of the based material between all three stain solutions.

Fig. 1

HAADF images of product A NPs (top) and energy dispersive X-ray spectroscopy (EDS) analysis (bottom) highlighting presence of both zinc oxide and iron oxyhydroxide particles

Fig. 2

HAADF images of product B including EDS analysis highlighting presence of both zinc oxide and iron oxyhydroxide particles

All stain solutions were also characterized via XAFS to elucidate zinc speciation in the pristine material to better understand transformation during incubation in SLFs. Following XAFS collection, a number of zinc standards were evaluated as potential models for speciation analysis. Five reference compounds were required to fit all of the data using a LCF approach. The five compounds included the following: zinc absorbed to Suwannee River humic acid (SRHA), an aqueous zinc citrate complex, aqueous zinc chloride, zinc oxide obtained from a secondary source, and zinc adsorbed to ferrihydrite (Zn-Fe2O3•0.5H2O) [21, 22]. Prior work confirms that the error in LCF results acceptably ranges between 5 and 10% [21, 22, 48].

The X-ray absorption near edge structure (XANES) region of the Zn k-edge for the three stains and ZnO are presented in Fig. 3. ZnO oxide has a very distinctive peak after the adsorption edge near 9680 eV. This strong feature may be used to identify the presence of ZnO in a sample. The XANES spectra for products A and B contain the distinctive feature and indicate ZnO is present in the sample. The relative abundance and speciation of the Zn species present were determined for each of the stains via LCF. Results from the LCF for the control stain indicates zinc species are distributed between the two organic complexes (68%), zinc adsorbed to ferrihydrite surfaces (28%), and ionic zinc (7%). LCF analysis of the XANES data for products A and B did not show a detectable quantity of Zn-Ferrihydrite. Instead, ZnO was present with similar contributions for zinc bound to organic material and ionic Zn (Fig. 3 and Table 1).

Fig. 3

XAFS spectra for an external ZnO reference, control stain (no NP additives), and commercial products. Both product A and product B show similar spectral features, with obvious similarities of the ZnO standard, as well. Spectra is pictured as a normalized and b first derivative spectra

ZnO transformation in water matrices

Initial studies were completed on pristine ZnO dispersed in Milli-Q water (pH 7), to obtain a baseline understanding of how SLF solution chemistry affects ZnO NP transformation. After dispersion, the resulting test solutions were further incubated in Milli-Q water (control), GS, and ALF. Figure 4 illustrates the effect of the different incubation fluids on the total concentration of suspended zinc in solution throughout sampling. As discussed in the “Material and methods” section, the samples were centrifuged prior to analysis to remove aggregates. For all samples, the highest concentrations of suspended zinc remaining in solution were seen within the first 72 h of incubation. Most of the zinc oxide dissolution and aggregation was observed for all samples within the first 3 h of incubation. Concentrations declined as fluid replenishment occurred throughout the 2-week incubation period. This decline was determined to be consistent with the anticipated effect of serial dilution due to incubation fluid replenishment. Incubation in GS correlated to similar results as the controls, 58 ± 1 ppm at 3 h, decaying to 31 ± 1 ppm after 72 h. Incubation in ALF, however, resulted in a threefold increase in the concentration of zinc remaining in the solution after centrifugation, with the highest concentration being 156 ± 9 ppm at 3 h, and decaying to 71 ± 3 ppm after 2 weeks. The changes in suspended zinc concentration are likely linked to the pH of incubation fluid.

Fig. 4

a Concentration of suspended zinc remaining in the water column of each sample after incubation of water dispersed ZnO NPs in GS and ALF. b The percentage of dissolved/ionic zinc (Zn passing through a 10 k-Da membrane

The total amount of zinc in solution was further separated into particulate and dissolved Zn via centrifugal filtration through a 100 kDa membrane (Fig. 4b). Comparisons for zinc dissolution were made at 72 h. Incubation in Milli-Q water (pH 7) was shown to cause little zinc dissolution, with 3.9 ± 0.1% being ionic at 72 h. Incubation in GS resulted in an increased amount of dissolved zinc, 32.1 ± 0.3% at 72 h indicating an interaction between salts in the GS and the suspended zinc that encourages dissolution. This is significant increase in dissolution in comparison to previous work by Adamcakova-Dodd et al. who reported less than 1% dissolution of ZnO over 2 weeks in GS [30]. Incubation in ALF resulted in total dissolution of suspended zinc within the first 3 h of incubation, supporting the conclusion that the low pH of ALF, compared to the more neutral GS, contributed to zinc dissolution. This complete dissolution on ZnO NPs in ALF is consistent with previously published work [30, 31].

ZnO transformation in stain-based matrices

ZnO NPs dispersed in the Control Stain before incubation are shown in Fig. 5. Initial concentrations of suspended zinc were similar regardless of whether the particles were dispersed in water (Fig. 4a) or stain (Fig. 5a). Relative to control incubations, stain dispersed particles incubated in GS showed a substantial in drop suspended zinc concentration during the incubation period. In control samples, the highest concentration was 59 ± 10 ppm at 3 h, decaying to 37 ± 5 ppm after 72 h. For the GS incubations, the highest concentration was 43 ± 6 ppm at 3 h, decaying dramatically to 6.4 ± 1 ppm after 72 h. The drop in suspended zinc concentration cannot be explained by the differences in pH between Milli-Q water and GS, 7 and 7.4 respectively. The addition of salts and polymeric substances in the GS appears to cause hetero aggregation with colloidal material in the stain decreasing the overall concentration of suspended zinc. Like dispersion in water, the highest concentration of zinc was found in samples incubated in ALF ranging from 151 ± 2 ppm at 3 h, decaying to 90 ± 3 ppm after 72 h.

Fig. 5

a Concentration of suspended zinc remaining in the water column of each sample after incubation of control stain dispersed ZnO NPs in GS and ALF. b The percentage of dissolved/ionic zinc (Zn passing through a 10-kDa membrane)

Subsamples of supernatant were again filtered through a 10-kDA membrane to determine the portion of dissolved zinc found in solution (Fig. 5b). Similar to the water dispersed samples, samples dispersed in stain and incubated in ALF had near immediate dissolution. Previous research had demonstrated that ZnO dispersed in stain was more resistant to chemical transformation compared to dispersion in water [22]; however, the low pH associated with the ALF appears to promote dissolution even in the more complex stain matrix. A substantial amount, 25.4 ± 0.1% of the total zinc incubated in GS was present as dissolved Zn at the 72-h sampling time, compared to controls, which showed no detectable dissolution after 72 h. Following the 72-h sampling, there is a noticeable decline in dissolved zinc, most likely due to the same stain-particle interactions as mentioned above, as well as the predicted dilution due to incubation fluid replenishment.

ZnO transformation in consumer products

Results from the SFL incubations for products A and B are presented in Fig. 6 and show that the concentration of suspended zinc in each incubation fluid was comparable between the two products even though product A had nearly twice as much Zn present (Table 1) and much smaller ZnO particles (Fig. 1) present. For both commercial products, incubation in ALF resulted in the highest concentration of total a Zn in solution and complete dissolution of ZnO after 3 h. This trend is consistent with results from incubation of ZnO NPs dispersed in the control stain (Fig. 5).

Fig. 6

a Concentration of suspended zinc remaining in the water column of each sample after incubation products A and B in water, GS, and ALF. b The percentage of suspended zinc determined to be ionic at each sampling point via centrifugation (10-kDa filter unit)

Additionally, there was no detectable dissolution of the ZnO for either product A or product B when incubated in water. Incubation of products A and B in GS resulted in a significant decrease in the total concentration of Zn present in solution compared to incubation in water. However, a large portion of this Zn was determined to be dissolved in nature, which was not observed in water incubations.

While the observed trends for both commercial products incubated in SLF were similar, there was one interesting difference observed during incubation in GS. After 72 h of incubation, Product A demonstrated a much larger portion of dissolved zinc (~ 68%) compared to product B (~ 34%). This is interesting as the trends for both water incubation and ALF incubation were similar for both products. It is likely that the difference in the size of the NPs in both commercial products play a role in this behavior when incubated in GS. Specifically, product B contains ZnO NPs (Fig. 2) of significantly larger size than those found in Product A (Fig. 1). Overall, data from commercial product incubations supports the conclusion that components within the SLFs, especially in GS, interact with ZnO NPs to cause aggregation and dissolution.

Incubation of ZnO, ZnO-spiked stains, and commercial products shed light on how the changes in the chemical composition of the incubation fluid impacts total Zn remaining in solution and the abundance of dissolved Zn. An increase in the quantity of dissolved Zn for the GS solution only occurred in the absence of stain (Fig. 4). In the presence of the stain, the GS total Zn concentration was substantially less than the water, indicating the GS solution likely resulted in aggregation of NPs present in the stains. Since the GS did not cause aggregation in the absence of the stain, it is likely the GS solution promoted aggregation of the nano-iron oxyhydroxide present which subsequently resulted in an aggregation of the ZnO NPs. Conversely, the low pH and citric acid present in ALF likely aided in promoting the dispersion and stability of nano ZnO.

Zinc speciation analysis

The speciation of Zn in each of the incubation samples was investigated through analysis of the Zn k-edge XANES. As described in the “Sampling procedure and analysis” section, material easily removed from solution via centrifugation was collected on 0.45-µm filters and analyzed to understand changes in zinc speciation in each SLF. A suite of standards was used in the analysis of collected samples. Zinc phosphate was used to represent the potential for interactions with phosphate in each system. Ionic zinc was represented by zinc chloride. Zinc bound to EDTA and Suwannee River Humic acid were used to model organic zinc complexes. ZnO was also included in analysis to represent non-transformed ZnO particles. Results from the LCF analysis of the samples after 3 h of incubation with the SLF solutions is presented in Table 2. It is critical to highlight that the values presented in table two do not represent the relative abundance of zinc in the entire incubation fluid. Rather, these values outline the zinc speciation in the heavy solids that were easily removed from solution during centrifugation after 3 h of incubation in each SLF.

Table 2

Percent of each species of zinc making up the suspended zinc in the water column of each incubation following 3 h. XAFS analysis represents results from linear combination fitting using the first derivate of normalized spectra from −20 eV below the edge to 60 eV above

	XAFS Analysis (Relative %)
	NP- Spiked Control Stain			Product A			Product B
	Milli-Q	GS	ALF	Milli-Q	GS	ALF	Milli-Q	GS	ALF
ZnO	100	100		49	-	-	16	-	-
ZnPO4	-	-	13	32	33	16	-	11	25
Organic	-	-	78	-	35	75	73	67	56
Ionic	-	-	9	19	32	9	11	22	19

The control system of 20 nm ZnO NPs dispersed in the control stain and incubated in water was found to be entirely ZnO in speciation. It is likely that other forms of zinc are present in the aggregates but not a concentration detectable by XAFS. The speciation of aggregated zinc was not altered when the particles were incubated in GS. However, Fig. 4 b clearly demonstrates some transformation of the ZnO NPs during incubation in GS through increasing concentration of dissolve zinc during incubation. ZnO was not present in the ALF solution instead zinc phosphate, ionic Zn, and Zn-organic complexes were identified confirming the incubation results (Fig. 6). Given the number of organic acids present in the ALF solution it is not surprising that the Zn-organic complexes dominate the speciation.

The product A and product B stains incubated in water, used as a baseline comparison, produced spectra indicating an initial mix of ZnO, ionic Zn, and ZnPO4 or organic Zn respectively. This speciation was shown to change dramatically during incubation in SLFs. Most noteworthy, ZnO in both products A and B is completely converted to other particulate species after only a 3-h incubation. The XAFS data collected herein clearly demonstrate the potential for rapid dissolution and transformation of ZnO in consumer products into dissolved, non-particulate zinc with multiple levels of complexity.

Conclusions

As nano-enabled products become increasingly prominent across industries, there is a growing need to better understand the potential health risks associated with application and use of products, including paints and stains. This study sought to investigate the behavior of ZnO NPs in SLFs to simulate the NP transformation associated with inhaling nano-enabled stains during application. The results of this study indicate that significant transformation of ZnO NPs in both laboratory produced, and commercially available products when incubated in SLFs. Additionally, the dissolution of ZnO into non-particulate species occurs at substantial amounts. Specifically, incubation of ZnO NP spiked control stains in GS resulted in approximately 25% of the added zinc determined to be dissolved after only 72 h. Dissolution of ZnO NPs in commercial products A and B was even higher during incubation in GS, 68% and 34% respectively. These results are especially interesting as previous work has not observed significant dissolution of ZnO NPs in GS [30, 31]. Critically, these studies did not include the presence of a secondary medium like a stain product. Clearly the addition of the stain components impacts the suspension and dissolution of ZnO during incubation in SLFs. It is also possible that ZnO NPs in commercial products undergo significant transformation before reaching the end user.

In all studies presented, herein, dissolution is enhanced in acidic conditions, including ALF, which mimics the intercellular environment of alveoli cells. Ongoing studies in this area should also consider the impacts of particle concentrations and morphology on the behavior of these products during exposure in both intended and foreseeable misuse scenarios. Furthermore, there is a great need to estimate true exposure of levels of NP additives during application and specifically identify what levels of NPs leave the sprayer nozzle and enter the breathing zone in the aerosol size range. Additionally, future work should be conducted on additional relevant metal oxide NPs, including CeO2 and TiO2. This study corroborates the potential concerns in inhalation and ingestion of NPs by consumers during the application, use, and disposal of ENM products and this topic should continue to be investigated in further research.

Below is the link to the electronic supplementary material.

Supplementary file1 (PDF 1030 KB)