Interior vs. Exterior Incorporation of Silver Nanoparticles in Cotton Fiber and Washing Durability

Materials

Bleached and desized woven cotton print cloth (98 g/m²) was purchased from Testfabrics, Inc. (Product No. 400, West Pittston, PA, USA). Mechanically, pre-cleaned raw white cotton fiber was acquired from T. J. Beall (Greenwood, MS, USA). Silver nitrate (AgNO₃, 99.9%) was purchased from J. T. Baker (Phillipsburg, NJ, USA). Sodium hydroxide (NaOH, 97%), ammonium hydroxide solution (NH₄OH, 28-30%), L-ascorbic acid (C₆H₈O₆, 99%), methyl methacrylate, butyl methacrylate, and methyl ethyl ketone were purchased from Sigma-Aldrich (St. Louis, MO, USA). All materials were used as received. Deionized (DI) water was also used.

Production of Interior Ag NPs

Interior Ag NPs were produced by the in situ synthesis of Ag NPs inside cotton fiber using a previously published method ²¹ with modification. Briefly, a cotton fabric was immersed in an aqueous solution of NaOH (20 wt%) for 15 min and passed through a laboratory padder (Werner Mathis USA Inc.). The resulting wet pick-up was 200% ± 5%. The alkali-swollen fabric was agitated in an aqueous solution containing AgNO₃ (1 mM) and NH₄OH (2.5 mM) and then passed through a laboratory padder to yield the wet pick-up of 150% ± 5%. The Ag ion-impregnated cotton was immersed in an aqueous solution of L-ascorbic acid (C₆H₈O_6, 0.1 mM), in which the fabric immediately turned yellow. The resulting interior NP-incorporated fabric was washed and air dried.

Production of Exterior Ag NPs

Exterior Ag NPs were produced by coating pre-synthesized Ag NPs onto a cotton fabric. The Ag colloidal solution was prepared by a previously published method ²² using raw cotton fiber. The unique surface chemistry of the raw cotton allowed the effective formation of Ag NPs not only inside the fiber but also on the surface of the fiber. Ag NPs formed on the fiber surface were collected by agitating the fabric in DI water at 40 ± 1 °C. The obtained dark yellow solution was filtered using Whatman No. 5 filtration paper followed by centrifugation (Ultracentrifuge, Hitachi) at 50,000 rpm for 30 min. The sedimented NPs were redispersed in DI water and diluted to a concentration of 100 mg/L. A cotton fabric was immersed in the obtained Ag colloidal solution for 15 min, passed through a laboratory padder to yield the wet pick-up of 100% ± 5%, and air dried.

Washing Experiment

The washing test was carried out using a Launder-Ometer (M228-AA, SDL Atlas LLC) based on AATCC TM61-2007, whose procedure is equivalent to five home or commercial laundering cycles, with modification. A stainless-steel canister (75 ± 5 mm in diameter, 125 ± 10 mm in height, 550 ± 50 mL) filled with 400 mL of water was preheated to 40 ± 1 °C in the Launder-Ometer. A fabric sample (5 × 5 cm) was placed in a 50-mL polypropylene bottle containing 15 mL of DI water or a detergent solution and 10 glass balls (5 mm in diameter). The detergent solution was prepared as a 0.15% w/v of a commercial laundry detergent (Tide, Procter & Gamble Co.) in DI water. The sealed bottle was placed in the preheated water in the canister and rotated at 40 ± 1 °C at a constant rate of 40 ± 2 rpm for 45 min. After completion of the washing procedure, the washing solution was collected, and the fabric was lightly rinsed with DI water and air dried. Washing in DI water without detergent was also conducted using the same procedure.

Characterization

The optical microscope images of cotton fabrics were obtained using a digital microscope (KH-8700, Hirox). The micrographs of the cotton fiber cross-sections were obtained using a transmission electron microscope (TEM) (JEM-2010, Jeol) operating at 200 kV. Cotton fibers were embedded in a mixture of methyl methacrylate and butyl methacrylate, which was polymerized in an ultraviolet (UV) cross-linker (UVP, CL-1000) for 30 min following published techniques.^22,23 A block of the fiber bundle was cut into ∼100 nm-thick slices using a PowerTome Ultramicrotome (Boeckeler Instruments Inc.). The obtained sections were placed on a carbon film-coated copper grid, and the embedding medium was removed using methyl ethyl ketone. Following the acquisition of TEM images, selected area electron diffraction (SAED) analysis was performed using the same accelerating voltage (200 kV). The average size and size distribution of Ag NPs were determined by measuring more than 250 particles using Image J software. ²⁴

The UV/Vis spectra were collected using a UV/Vis/NIR spectrometer (ISR-2600, Shimadzu). A quartz cuvette with a 1-cm optical path length was used for solution samples. An integrating sphere unit was used for cotton samples. Absorbance spectra were collected over a wavelength range of 220-800 nm.

The amount of Ag was determined using ICP-MS in the ICP-MS Metals Laboratory at the University of Utah. For solution samples, a 4-mL solution was dried down and refluxed for 1 h with 2 mL of 16 M nitric acid on a hot plate and dissolved in 1 mL of 0.8 M nitric acid with 10 ppb of indium. For cotton samples, ∼0.05 g of the sample was treated with 2 mL of 16 M nitric acid (trace metal grade) and digested in a Milestone Ethos microwave system. The digest was diluted by weight 1:10, and 10 ppb of indium as an internal standard was added. The digested solution concentrations were determined by an external calibration curve, which was obtained using a silver single element standard (Inorganic Ventures).

The concentration of Ag ions in washing solutions was determined using a silver/sulfide ion-selective electrode (ISE, ISE-8755, Omega Engineering Inc.). Before measurement, a daily calibration curve was established using silver standard solutions. Silver standard solutions were prepared from silver nitrate in the silver ion range from 0.9 to 37 μM in DI water. A total of 1 mL of an ionic strength adjuster, sodium nitrate (20 mM), was added in silver standard solutions and washing solutions. Concentrations of Ag particles less than 1 nm in size were determined using ultrafiltration/ICP-MS. The washing solution was filtrated using a centrifugal unit (Sartorius Stedim Vivaspin6, Sartorius AG) equipped with a poly(ether sulfone) membrane at a molecular weight cutoff of 3 kDa (ca. 1 nm). The filtered solution was analyzed by ICP-MS.

Interior and Exterior Ag NPs

Figs, 1a and b show the optical microscope images of cotton fabrics containing interior and exterior Ag NPs, respectively, obtained using different magnifications. The cotton fabric with interior NPs exhibits a noticeable yellow color, indicating the incorporation of a large amount of NPs by the in situ synthesis. The unique optical property of Ag NPs results from surface plasmon resonance (SPR), which is induced by the interaction of conduction electrons of the NPs with the electromagnetic wave of incoming light. It can also be seen that the fabric structure with interior NPs is denser than that with external NPs. This is because the internal synthesis involves an alkaline treatment that swells cotton fibers laterally. The increased yarn thickness is observable in its higher magnification image.

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Fig. 1

Optical microscope images of cotton fabrics containing (a) interior and (b) exterior Ag NPs taken at different magnifications.

To verify the interior and exterior incorporation morphologies, the cross-section of cotton fiber was examined by TEM. Figs. 2a and b show the TEM images obtained after the in situ synthesis of Ag NPs within the fiber and after coating of Ag NPs onto the fabric, respectively. The insets show the corresponding low-magnification TEM images and indicate the sections taken at high magnification highlighted in red circles. In line with the earlier microscope image, the internal synthesis transformed the bean-shaped cotton fiber into a rounder shape. In coating, a smooth surface is expected to be less favorable for the physical adsorption of NPs as compared with a rough surface. It is also expected that the NPs coated on a smoother surface are more easily washed of than those deposited on a rough surface. The alkaline treatment used in the internal synthesis also caused the rearrangement of cellulose chains from a parallel manner (cellulose Iβ) to an anti-parallel manner (cellulose II). ²¹ Such conversion increases the number of available hydroxyl groups,^25,26 which may have an affinity with Ag ions present on the NP surface. However, these electrostatic interactions are expected to be too weak to alter the washing durability of NPs.

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Fig. 2

TEM micrographs of the cross-sections of cotton fibers containing (a) interior and (b) exterior Ag NPs. The insets show the low-magnification images and the sections taken at the high magnification highlighted in the red circles. Size histograms are shown for (c) the interior Ag NPs and (d) the exterior Ag NPs.

The high magnification image shows that the in situ synthesis produced Ag NPs in the interior of the fiber. The NPs were uniformly dispersed throughout the cross-section of the fiber. For the Ag NP-coated sample, most NPs were located on the edge of the cross-section of the fiber. It can be seen along the surface that some NPs were aggregated into a large particle. Both interior and exterior NPs were quasi-spherical. Figs. 2c and d show the histograms of the diameters of interior and exterior NPs, respectively. The size of interior NPs was more narrowly distributed, whereas the size of exterior NPs was right-skewed due to the presence of a few large particles. The average diameters of interior and exterior NPs were 11.2 ± 2.8 and 14.7 ± 7.6 nm, respectively. The polydispersity indices of interior and exterior NPs were 0.06 and 0.27, respectively. The larger average size and polydispersity of exterior NPs were attributed to particle aggregation during treatment. Other studies also observed inhomogeneous dispersion and particle aggregation from the bulk solution treatment of NPs. The particles observed on the surface of textiles were up to two orders of magnitude larger than the pristine particles in the colloidal solution of treatment.^27-29 Figs. 3a and b show the SAED patterns of interior and exterior NPs obtained from the cross-section of cotton fibers, respectively. Both particles exhibit four concentric rings with intermediate bright dots, which correspond to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of the faced-centered cubic (fcc) structure of elemental Ag.

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Fig. 3

SAED patterns of (a) interior and (b) exterior Ag NPs taken from the cross-section of cotton fibers. The concentric rings with intermediate bright dots representing the lattice planes of elemental Ag are highlighted in red.

Fig. 4a shows the UV/Vis absorbance spectra of cotton fabrics containing interior and exterior Ag NPs. These Ag NP-treated cotton fabrics developed a single absorbance peak resulting from the SPR of NPs. The untreated cotton fabric did not show any recognizable peaks over the entire wavelength range studied (not shown here). The SPR peak of interior NPs was centered at ∼427 nm (λ_SPR), whereas the λ_SPR of exterior NPs was ∼420 nm. The SPR intensity (I_SPR) of interior NPs was much higher than the intensity of exterior NPs. As I_SPR is positively related to the concentration of NPs, the observed higher I_SPR indicates the incorporation of a greater amount of interior Ag NPs. To verify this, the contents of Ag-based on the dry weight of cotton fabrics were measured using ICP-MS (Fig. 4b). The concentration of interior Ag NPs was about five times greater than that of exterior Ag NPs. It was also found that the measured concentration of exterior Ag NPs (41 mg/ kg) was much smaller than their calculated concentration (100 mg/kg) based on the concentration of the Ag colloid and the applied wet pick-up. This discrepancy indicates that a significant amount of Ag NPs was lost during the treatment process (i.e., the adsorption onto the metal vessel and padder).

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Fig. 4

(a) UV/Vis absorbance spectra of cotton fabrics containing interior and exterior Ag NPs. (b) Concentrations of interior and exterior Ag NPs based on the dry weight of the cotton fabric measured by ICP-MS.

Analysis of Washed Cotton Fabrics

The washing durability of interior and exterior NPs was examined by monitoring the changes in the UV/Vis spectra of cotton fabrics. Figs. 5a and b show the percentage I_SPR with increasing simulated home laundering cycles in water and detergent solution, respectively. In water, the I_SPR of cotton fabrics containing interior and exterior NPs dropped after the initial washing of five laundering cycles; their respective I_SPR losses were 6% and 17%, respectively. As the laundering cycles increased, the I_SPR for interior NPs remained relatively steady, whereas the I_SPR for exterior NPs gradually decreased. In the detergent solution, the effect of the incorporation method was more pronounced. The I_SPR losses for the interior incorporation were not greatly influenced by detergent (i.e., similar (slightly greater) level to the losses in water). However, the I_SPR losses for the exterior incorporation were greatly increased in the detergent solution, indicating that exterior NPs are more vulnerable to detergent solution. The I_SPR for the exterior incorporation continuously dropped for the extended laundering; the I_SPR losses after 50 cycles were 22.5% in water and 34.8% in the detergent solution. The corresponding values for the interior incorporation were 7.5% and 8.3%.

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Fig. 5

Percentage intensities of surface plasmon resonance (SPR) peaks (I_SPR) of cotton fabrics containing interior and exterior Ag NPs washed in (a) water and (b) detergent solutions as a function of the simulated home laundering cycle. Wavelengths at the maximum SPR intensity (λ_SPR) of cotton fabrics containing interior and exterior Ag NPs washed in (c) water and (d) detergent solutions as a function of the simulated home laundering cycle.

It was also noted that the λ_SPR of cotton fabrics containing interior and exterior NPs varied with the laundering cycle. Figs. 5c and d show the plots of λ_SPR measured after washing in water and detergent solution, respectively, as a function of the simulated home laundering cycle. The variation patterns of λ_SPR were consistent for water and detergent solutions. The λ_SPR for interior NPs slightly increased, whereas the λ_SPR for exterior NPs decreased, as the simulated home laundering cycle increased. These different trends for the interior and exterior systems can be explained by their distinctive Ag release behaviors. Upon exposure to water, the NPs enclosed in the interior of cotton fiber release Ag ions through the dissolution and/or oxidation of Ag on the particle surface. These Ag ions diffuse to the surface of cotton fiber and are adsorbed onto NPs that are present on the fiber surface. The adsorption of Ag ions on the surface of Ag NPs causes the redshift of λ_SPR. In the detergent solution, the Ag ions could react with negatively-charged detergent components. For the exterior incorporation, large or aggregated NPs were readily detached from the fabric surface as laundering was continued. The smaller NPs remaining on the fabric resulted in the blueshift of λ_SPR.

The change in the concentration of Ag NPs in the cotton fabric after washing was quantified using ICP-MS. Figs. 6a and b show the percentage of Ag contents remaining (or Ag loss) in the fabrics after incremental simulated home launderings in water and detergent solution, respectively. These results were compared with the I_SPR results (Figs. 5a and b), which reflect the concentration of Ag NPs near the fabric surface. For the fabric with interior NPs, the Ag loss patterns in water and detergent solution were similar to the I_SPR loss patterns, with slightly greater magnitudes. However, the fabric with exterior NPs exhibited Ag loss behavior different from that of I_SPR. The Ag losses in water and detergent solution steeply increased until 10 laundering cycles, although the increases of the I_SPR losses diminished above 5 cycles. In terms of magnitude, the Ag losses were much greater than the I_SPR losses. The exterior system continuously lost Ag with increasing the number of laundering cycles, resulting in that only 37% and 22% of the original amount of Ag remained after 50 cycles in water and detergent solutions, respectively. The respective values for the interior system were 84% and 76% of the original Ag amount. The consistent results of the bulk and surface analyses for the cotton fabric containing interior NPs are attributed to their unique incorporated structure (i.e., uniform dispersion of NPs throughout the entire volume of cotton fiber). These characterizations agreed on the improved washing durability by the interior incorporation.

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Fig. 6

Percentage of Ag remaining in cotton fabrics containing interior and exterior Ag NPs after incremental simulated home launderings in (a) water and (b) detergent solutions.

Analysis of Washing Solutions

The analyses of the washed cotton fabrics indicate that the washing solutions may contain NPs. Washing solutions were collected after incremental laundering cycles and analyzed using various analytical methods. Figs. 7a and b show the stacked UV/Vis absorbance spectra of water obtained after washing cotton fabrics containing interior and exterior Ag NPs, respectively. The washing water for the interior system did not develop any noticeable SPR peaks for all laundering cycles studied. The cotton fabric with interior NPs released Ag NPs below the detection limit of UV/Vis spectroscopy and/or a different form of Ag (i.e., ionic Ag). On the other hand, the spectra for the exterior system clearly showed SPR peaks. The well-defined SPR peak appearing even after the initial washing indicates that exterior NPs were easily detached from the fabric. This strong SPR peak was further developed with increasing laundering cycles.

Figs. 7c and 7d show the spectra of detergent solutions for the interior and exterior incorporations, respectively. Similarly, the absorbance spectra for the interior incorporation, which were identical for all laundering cycles, exhibited no evident SPR peaks. The peak near 260 nm and a shoulder at around 320 nm were from detergents. The absorbance spectra for the exterior incorporation developed SPR peaks with higher intensities, indicating the higher concentrations of Ag NPs in the detergent solution than in water.

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Fig. 7

UV/Vis spectra of water collected after incremental simulated home launderings of cotton fabrics containing (a) interior and (b) exterior Ag NPs. UV/Vis spectra of detergent solutions collected after incremental simulated home launderings of cotton fabrics containing (c) interior and (d) exterior Ag NPs.

Fig. 8a shows the I_SPR of washing solutions for the exterior incorporation as a function of the simulated home laundering cycle. Agreeing with the I_SPR results of the washed fabrics (Figs. 5a and b), the major increases in I_SPR of water and detergent solutions occurred after the first 5 cycles, and the I_SPR of the detergent solution was higher than that of water. These results support the previous conclusion that exterior NPs were mostly detached during the initial washing, and detergent facilitated this leaching process. Fig. 8b shows the effect of the laundering cycle on the λ_SPR of washing solutions for the exterior incorporation. The λ_SPR (421 nm) of water at 5 cycles was comparable with the λ_SPR (420 nm) of the unwashed cotton fabric (Fig. 4a), but it gradually decreased with increasing laundering cycles. Upon extended exposure to water with agitation, the surface of the particles was dissolved. The λ_SPR of the detergent solution exhibited a blueshift to the λ_SPR of the unwashed cotton fabric and remained constant throughout the laundering cycles studied. The components of detergent, which can chelate NPs and form Ag complexations, interrupted the aggregation and dissolution of the particles.

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Fig. 8

(a) Intensities of the SPR peaks (I_SPR) and (b) wavelengths at the maximum SPR intensity (λ_SPR) of washing solutions collected after incremental simulated home launderings of cotton fabrics containing exterior Ag NPs.

Textiles treated with Ag NPs, if they are weakly bound, easily leach particulate Ag during laundering that causes various mechanical impacts such as centrifugal, frictional, and gravitational forces. These forces also facilitate the dissolution of Ag on the surface of NPs (either bound or free) and subsequent release of ionic Ag into washing solutions. To examine such transformation of Ag, washing solutions were further analyzed using ICP-MS and ultrafiltration. Figs. 9a and b show the concentrations of total Ag and Ag smaller than 1 nm in washing water, which were measured before and after ultra-filtration, respectively, as a function of the simulated home laundering cycle. Although the concentration of interior NPs incorporated was about five times greater than that of exterior Ag NPs coated (Fig. 4b), the amounts of total Ag detected in water were smaller for the interior incorporation. The total Ag concentration from the interior system continuously increased with subsequent laundering cycles, whereas the concentration from the exterior system reached its maximum at 10 cycles and depended less on additional cycles. For the case of Ag < 1 nm (Fig. 9b), the opposite result was observed (i.e., higher concentrations for the interior incorporation as compared with those for the exterior incorporation). When taken together, these two measurements indicate that the interior incorporation allowed the continuous, low-level Ag release, with a large portion being ionic Ag. To confirm this, the size fractions of Ag in water for the interior and exterior incorporations are compared, as shown in Figs. 9c and d, respectively. As expected, the fraction of Ag < 1 nm was greater for interior incorporation throughout the extended laundering. The majority, above 80%, of Ag released from the exterior system was in a particulate form.

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Fig. 9

Concentrations of (a) total Ag and (b) Ag smaller than about 1 nm in water measured by ICP-MS after incremental simulated home launderings of cotton fabrics containing interior and exterior Ag NPs. Corresponding Ag size fractions for (c) interior and (d) exterior incorporations.

Figs. 10a and b show the concentrations of total Ag and Ag < 1 nm in the detergent solution, respectively. Compared with the result for water (Fig. 9a), the concentrations of total Ag for the exterior incorporation greatly increased, while those for the interior incorporation remained at similar levels. As a result, the incorporation method exerts a more pronounced effect in contaminating a detergent solution than in water. There was about a threefold reduction in the concentration of Ag of the detergent solution when switching from exterior to interior systems, even with the original amount of interior NPs being fivefold greater than the exterior NPs. On the other hand, the concentration of Ag < 1 nm appeared to be independent of the incorporation method (Fig. 10b). It was also found that the concentrations of Ag < 1 nm in the detergent solution were lower than those in water regardless of the incorporation method. These distinctive phenomena for the detergent solution were more obvious by the size fractionation of Ag, as shown in Figs. 10c and d for interior and exterior incorporations, respectively. The fractions of Ag < 1 nm for both interior and exterior incorporations greatly dropped in the detergent solution. Only less than 20% and 10% of the total Ag remained as an ionic form for interior and exterior incorporations, respectively. In combination with earlier results, this result suggests that although the interior and exterior incorporations induced different Ag release behaviors (i.e., the predominant release of ionic Ag by the interior incorporation), most of the Ag released into the detergent solution underwent the chemical transformation into a particulate form. The effect of the laundering cycle on this transformation was not noticeable.

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Fig. 10

Concentrations of (a) total Ag and (b) Ag smaller than about 1 nm in detergent solution measured by ICP-MS after incremental simulated home launderings of cotton fabrics containing interior and exterior Ag NPs. Corresponding Ag size fractions for (c) interior and (d) exterior incorporations.

ICP-MS with ultrafiltration measures Ag that passed through the 3 kDa membrane, so it is assumed to measure Ag ions. Strictly, this measurement also includes sub-nanometer Ag particulates. The concentration of Ag ions alone was independently measured using an ISE. Figs. 11a and b show the concentrations of Ag ions in water and detergent solutions, respectively, as a function of the simulated home laundering cycle. The plots all showed a similar trend that the highest concentrations occurred at 5 cycles, followed by lowered concentrations with less dependence on additional cycles. This was different from the trend of increasing concentration for Ag < 1 nm observed by ICP-MS/ultrafiltration. The ionic Ag initially released could be adsorbed onto the surface of free NPs or oxidized. Such various routes of Ag fate are considered for the discrepant functions between the laundering cycle in the ICP-MS and ISE results. However, these analyses agreed that the fabric containing interior NPs released more Ag ions than the fabric containing exterior NPs. They also agreed that the Ag transformation was overtaken by the interactions with detergent components, resulting in less effect of the incorporation method on the concentration of Ag ions in the detergent solution.

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Fig. 11

Concentrations of Ag ions in (a) water and (b) detergent solution measured by ISE after incremental simulated home launderings of cotton fabrics containing interior and exterior Ag NPs.