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Polymers 2019, 11, 2000 11 of 21 indicates that the amount of impregnated/coated material is insufficient for a strong signal. However, silver-containing samples display a large signal enhancement, which is due to AgNP hot spots, i.e., it is a surface enhanced Raman spectroscopy (SERS) effect, and serves to mask most other signals [121–123]. The non-uniform enhancement of the respective Raman bands is a feature of SERS and makes it difficult to extract further ID/IG data. In addition, to the main D and G bands, a sharp, clear band is also observed at ~1720–1730 cm−1 for most samples and is mainly due to the asynchronous stretch of C=O. The easy hydration reaction between the molecules at C=O means such vibrations are easy to disturb due to the high concentration of electron–hole pairs present [124,125]. Such a signal identification is in good agreement with the ATR-FTIR data. The decreasing signal intensity observed with higher NaBH4(aq) concentration indicates that bonding is facilitated, at least in part, via reduction of carbonyl groups. The C=O signal enhancement is dependent on the short distance and subsequent orientational interaction between AgNP and carbonyl groups. As such, it seems to not have been a common feature of past reports to date simply because the nature of the non-conventional experimental synthesis method utilised in this paper, whereby the reducing agent impregnates the substrate first, prior to the Ag/Ag-rGO being incorporated in situ, which allows for a unique surface configuration to be achieved that seemingly facilitates the interaction of AgNP with the surface carbonyl groups, yielding the eventual signal enhancement. ATR-FTIR spectra (Figure 4) include readings for samples where data were collected both before and after washing cycles, to investigate the structural changes in the impregnated fabrics as well as probe the interfacial interactions. In all cases, sample spectra were dominated by signals from the polyviscose substrate, accompanied by certain weaker composite-based signals, which indicate the relatively low loading of additive in all cases. Common signals for all samples, except the solely rGO impregnated samples, are the presence of hydroxyl stretching vibrations (~3350 cm−1), carbonyl stretches (~1745 cm−1), and bands ascribed to C–O bending and stretching vibrations of hydroxyl groups (1300–1400 cm−1 and ~1048 cm−1, respectively) [126–128]. The broad band observed at ~1220 and ~1110 cm−1 were assigned to C–O–C/ether from epoxy groups of the substrate, in addition to a common O–C=O signal (1650–1750 cm−1) [129,130]. The solely rGO samples had heavily reduced peak intensities for signals related to O-containing functional groups after reduction, which indicates strong and efficient interaction and conversion of the GO with the previously available hydroxyl and carbonyl type binding sites. The substrate is, thus, highly saturated. A new peak at ~1548 cm−1 due to sp2-hybridised C=C (in-plane stretching), confirms the effective reduction of GO. The strong signal is perhaps indicative of the higher relative percentage of C–C bonds from rGO incorporation and overall loading into the substrate [131]. The C–O stretching and O–H deformation vibration of carboxylic groups are represented by the peak at ~1401 cm−1 [132]. The comparatively lower content of oxygen functional groups present indications of involvement of O–H groups in the reduction to AgNPs [133,134]. The fact that signals are not completely absent in the equivalent AgNP and Ag-rGO combinations suggest that such substrates may not be fully saturated, which indicates only a moderate reduction of Ag+ to AgNP, perhaps due to a less efficient chemical conversion process, or perhaps (in the case of Ag-rGO) competitive formation effects during the simultaneous transformation of GO to rGO. After accelerated laundering, all samples indicate structural changes, with greater resemblance toward the original blank substrate profile. This indicates loss and removal of the impregnating material, with the lowest effect (i.e., least signal attenuation) seemingly observed for Ag-rGO samples. 3.3. Antimicrobial Properties Testing The antimicrobial efficacy of the various nanocomposite medical textiles was tested against E. coli. Results reveal that Ag-rGO composites exhibit enhanced antibacterial activity (near 100% suppression for 6 h treatment, against an untreated ‘control’ blank substrate sample) as compared to solely AgNP incorporated samples (98.3 %) although a significant reduction of bacterial growth was observed in both cases (Figure 5) [135]. A reduction in the numbers of viable bacteria was observed after 6 h incubation on the treated sample substrates, in the dark, with statistically significantPDF Image | Antimicrobial from Silver-Graphene Coated Medical Textiles
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