Gold nanoparticles driven self-assembling hydrogel via Host–Guest system

Journal of Molecular Structure 1200 (2020) 127063

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Gold nanoparticles driven self-assembling hydrogel via HosteGuest system Lamia L.G. Al-mahamad Department of Chemistry, College of Science, Mustansiriyah University, Baghdad, Iraq

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2019 Received in revised form 19 August 2019 Accepted 10 September 2019 Available online 11 September 2019

In this work, a new class of host-guest polymer has been synthesised and characterized based on preparation Au(I):6-thioguanosine hydrogel (host) that is carrying gold nanoparticles AuNPs (guest). The morphology of the formed polymer was investigated with Atomic Force Microscopy (AFM) technique. The images displayed converting the linear polymer of the hydrogel to the stars-like shape upon treating with AuNPs. Statistical analysis was achieved for the AFM images to investigate the height of the polymer. The data revealed that the height fibre of the new polymer was ~1.5 nm with presence new heights in the range of 8e15 nm which are belong to the entities of AuNPs in the gel. Also, the new polymer exhibits typical diameter (1.5 nm) which is lower than the diameter of the polymer without AuNPs. The presence of AuNPs causes quenching the fluorescence of the hydrogel from 606 nm to 511 nm upon treating with AuNPs. These observations confirm the effect of intramolecular interactions of AuNPs on the morphology and the optical properties of the Au(I) hydrogel. © 2019 Elsevier B.V. All rights reserved.

Keywords: Host-guest system Gold nanoparticles Hydrogel 6-Thioguanosine Linear polymer Quenching

1. Introduction Gold nanoparticles (AuNPs), and in particular spherical nanoparticles, are characterized with multiple surface functionalities that establish a good base for assembling these particles with different biological materials [1e3]. The special properties of AuNPs give rise to use these particles in different applications in biomedical fields by exploiting the size and the shape of these particles in this field such as sensing, therapeutic agent processes, and imaging [3,4]. The combination of AuNPs with hydrogel structure is promising for the synthesis of nanomaterials with desirable functions and excellent performances due to combining the large surface area, the small size of NPs, high porosity, quenching fluorescence, etc. with the hydrophilic structure of the hydrogel, and making perfect platform of these nanomaterials for using in different applications such as filtration, tissue engineering, drug delivery, and nanotechnology [5,6]. Polymers are important hosts for NPs due to the flexibility feature of these entities that allows control the electric conductivity between the NPs and the semiconductor materials via electron tunnelling and variable range hopping (VRH) [7]. In addition, the characteristic plasmonic of NPs provides strong absorption for different compounds, and such

E-mail address: [email protected]. https://doi.org/10.1016/j.molstruc.2019.127063 0022-2860/© 2019 Elsevier B.V. All rights reserved.

specific properties give attention for widely using these nanomaterial in different applications, especially in sensing [8] and surface enhance Raman scattering (SERS) [7,9]. Several conditions must be achieved to make the interaction between the polymer and the NPs: first, the NPs should exhibit strong affinity towards the chains of the polymer [10], second, lots of interaction must be occurred between the chains of the polymer and the NPs to perform the percolation of the network, and finally, the diameter of the NPs should be less than the polymer length [11]. Sainudeen and et al., had decorated MgO nanoparticles on MgO nanofibres to control the shape and the diameter of the resulting fibres [12]. Esra and et al. reported preparing Fe3O4 nanorod that composite poly Nisopropyl acrylamide (PNIPA), the composite nanomaterial showed fast catalyst behaviour towards lysozyme, which gives this material the potential to use in the biomedical field [13]. Emulsion polymerization also use to prepare dye-loaded polymer NPs to gain the fluorescent of nanoparticles which are brighter than organic dyes to apply in vitro and in vivo imaging [14]. Another advantage for composing NPs with polymers can be exploited by synthesis composite polymer with small diameter, for example, a small diameter nanofibre (1.2 nm) which was synthesised for nylon-4,6 polymer by adjusting the concentration of this polymer using electrospinning method [15]. This technology found rapidly growing by researchers to prepare nano fibres for different materials with very small diameters [16]. Guanosine nucleoside and its

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derivatives, such as 6-thioguanosine, shows high affinity towards coinage metal ions that includes gold, silver, and copper by coordination reactions via the active binding sites of these nucleosides and forming nano fibrous structures and nano filament structures with many microns in length [17,18]. Herein, the preparation and characterization of self-healing hydrogel driven by intramolecular interactions with AuNPs is reported. The specific features of NPs offer a good platform to compos these nano structures with fibrous polymers to obtain nanofibres possess small diameter, new morphology and quenching fluorescence properties. 2. Experimental 2.1. Chemicals and materials All chemicals were purchased from Sigma Aldrich and were used as received without further purification. Tetrachloroauric acid (HAuCl4.3H2O), 6-thioguanosine, sodium citrate HOC(COONa)(CH2COONa)2$2H2O, and 2,20 -thiodiethanol (HO(CH2)2S(CH2)2OH). 2.2. Au nanoparticles preparation Gold nanoparticles (AuNPs) were prepared by reducing the solution of HAuCl4.3H2O (concentration 4.24  106 mol) with a sodium citrate solution [19]. Cary 100 Bio UVeVisible Spectrophotometer was used to measure the absorption of the red colour solution of AuNPs. The morphology of the particles was investigated by transmission electron microscopy (TEM). The sample was prepared by drop-casting 1.5 mL of AuNPs onto a carbon coated copper grid substrate, left to dry by air prior to scan. Image J software was used to estimate the particle size of the particles. 2.3. Gel preparation with Au nanoparticles Gold gel was prepared in accordance with the procedure in the literature [18], by reaction equimolar equivalents of Au(I) ions with 6-thioguanosine nucleoside in aqueous solution. A typical sample of the gel with AuNPs was prepared by adding the solution of HAuCI4 (13 mg, 0.033 mmol), formed by reduction of HAuCl4 with two equivalents of 2,20 -thiodiethanol (HO(CH2)2S(CH2)2OH), to the solution of 6-thioguanosine (10 mg, 0.033 mmol). To this mixture, 200 ml of 4.24  106 mol of AuNPs solution was added, immediately. Upon shaking the mixture, yellow gel was formed with thick upper layer, which extending inside the gel along the vial with time. 2.4. Atomic force microscopy (AFM) measurements AFM measurements were carried out using a Multimode 8 atomic force microscope with a NanoscopeV controller (Bruker), and an ‘‘E’’ scanner. The data were acquired using NanoScope Analysis 1.5 software (Bruker). P-silicon (100) wafers were used to achieve AFM measurements by adding 2 mL of the sample onto a clean silicon wafer and drying by air. 3. Results and discussion 3.1. Au(I):6-thioguanosine hydrogel preparation with AuNPs The linear fibrous morphology for Au(I):6-thioguanosine hydrogel affords a good host for AuNPs to take positions between the linear chains of the polymer to form supramolecular hydrogel loaded with AuNPs with stars-like morphology [20]. Forming the new morphology depends on the volume of the AuNPs that used; in

case of using a small volume (80 mL AuNPs/1 mL of the hydrogel) no change in the linear morphology has been seen, however, using 200 mL of AuNPs revealed changes in the morphology of the fibres from linear to star-like shape. The changing in the morphology can be attributed to the intramolecular interactions, which is favoured in the linear structures, between the AuNPs as (guest) and Au(I):6thioguanosine molecules in the polymer as (host) and this confirms efficient binding in host-guest system, and as a consequence, no aggregation was formed for AuNPs which is usually can find with polymers that have branched structures [21]. AFM technique was used to investigate the new morphology of the formed hydrogels that carried AuNPs. In addition, statistical analyses were also achieved to find the difference in the heights and the diameters of the hydrogel before and after treating with AuNPs. Fig. 1 shows a schematic illustrates the method for preparing Au(I) hydrogel with and without AuNPs and displays the role of AuNPs to alter the linear morphology of the Au(I) hydrogel. 3.2. Transmission electron microscopy (TEM) characterization of AuNPs TEM images were obtained by scanning the sample of AuNPs using Philips CM100 electron microscope at accelerating voltage 100 kV. The sample was prepared by dropping 1.5 mL of AuNPs onto a carbon coated copper grid substrate, left to dry by air prior to scan. The images showed that the morphology is mostly of monodispersed spherical shapes, as shown in Fig. 2(a). The particle size distribution, which was analysed using Image J software, gave a range of 1e22 nm and the data demonstrated that the main diameter was 7 nm and the average diameter was 3.5 nm, as shown in Fig. 2(b). The TEM image shows a very good evidence that the fabricated AuNPs are less aggregations, and this can give a rise to predict the appearance of only one plasmon peak at around 530 nm and no secondary plasmon peak will appear at a longer wavelength (longer than 600 nm). The role of the aggregated NPs in the appearance of secondary plasmon peak can be attributed to the delocalization of free-electrons in the aggregated NPs as the aggregation can occur in the region with energy lower than 2.5 eV (~wavelength > 500 nm) where the effect of Drude absorption, which associates with increasing the concentration of free electrons in the AuNPs, is dominated in the infrared region [22,23]. 3.3. UVevis characterization Reducing the solution of HAuCl4.3H2O with a sodium citrate solution leads to form a red colour solution of AuNPs. The Absorption spectrum of this solution, Fig. 3(a), was measured by using UVeVis technique. The measurements displayed formation single surface plasmon resonance (SPR) peak at 530 nm with a half width at full maximum (HWFM) ~ 53 nm which usually reveals with red coloured solutions of AuNPs, and this indicates formation of monodispersed colloidal spherical of AuNPs [24,25]. The absorption spectrum of Au(I):6-thioguanosine hydrogel, Fig. 3 (b), shows formation absorption band around 360 nm which can be attributed to metal-ligand charge transfer. Upon preparing the Au(I) hydrogel that composed AuNPs, the exocyclic amino group of 6thioguanosine on the side chains of the polymer was protonating and forming ammonium ions which adsorbed on the surface of the AuNPs and this leads to form greater size of AuNPs which causes a red shift in the SPR band for AuNPs from 530 nm to 565 nm, another important reason to occur this shifting can be assigned to change the dielectric constant of the AuNPs as a result of perturbation of the electrical double layer that surrounding the AuNPs upon adding to the hydrogel, as shown in Fig. 3 (c) [2,26e28]. Fig. 3(d) is a small area of (c) showing clearly the red shift that

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Fig. 1. Schematic illustration method formation host-guest system between Au:6-thioguanosine fibres and AuNPs. (a) AFM image displays one-dimensional fibres [18] extending many microns in length with scale bar 1 mm for Au(I) xeogel before treating with AuNPs. (b) AFM image for Au(I) xerogel with AuNPs displays formation of star-like shape morphology, the scale bar was 500 nm.

Fig. 2. (a) TEM image of AuNPs with scale bar 2 mm showing the special morphology of AuNPs. The image was analysed using Image J software to estimate the diameter. Analysis the particle size distribution gives a range of (1e22 nm) and the main diameter was 7 nm, while the average diameter was found 3.5 nm.

occurred in the surface Plasmon resonance (SPR) of AuNPs. 3.4. Fluorescence spectroscopy The fluorescence spectrum of Au(I):6-thiogunosine hydrogel in Fig. 4(a), with excitation 360 nm, showed a broad emission peak covering the area between 520 nm and 720 nm and centred at 606 nm [18]. While the fluorescence spectrum of the hydrogel with AuNPs in Fig. 4(b) displayed main luminescence peak around 490 nm with a shoulder band around 510 nm. The appearance of the shoulder band is probably assigned to the aggregation of the polymer chains [26]. In addition, this shoulder can be assigned to the difference in the torsion angles that occurred in two molecules of the polymer that resulted from the difference in the degree of the

conjugation of exo amino groups with p-p system [29]. The quenching in the fluorescence of the hydrogel upon treating with AuNPs from 606 nm to 490 nm can be assigned to the effect of dipole-dipole interactions between the AuNPs and the molecules of €rster Au(I):6-thioguanosine hydrogel. This effect is known as Fo resonant Energy Transfer (FRET). Another reason for occurring the quenching can be attributed to the distance between the AuNPs and the molecules of hydrogel, as the quenching increases when the distance is very close and vice versa. This effect, which is called as Dexter decay, can lead to change the rate decay of the molecules and causing quenching in the fluorescence of the hydrogel that treated with AuNPs [30,31]. The blue shift in the quenching florescence can be assigned to the overlap of the absorption wavelength of AuNPs and the Au(I) hydrogel, which can make

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Fig. 3. UVeVis spectra of: (a) red colour solution of AuNPs showing the surface plasmon resonance (SPR) of the single peak at 530 nm suggests formation a monodispersed of colloidal spherical AuNPs. (b) The absorption spectrum of Au(I):6-thioguanosine hydrogel, the band around 360 nm is due to metal-ligand charge transfer. (c) Optical absorption spectrum of Au(I) hydrogel carrying AuNPs shows the absorption band of the gel at 360 nm and the red shift for the SPR band of AuNPs from 530 nm to 565 nm which is assigned to the formation of the greater size of AuNPs in the hydrogel [26]. (d) Small area of (c) displays the red shift in the SPR band of AuNPs.

Fig. 4. (a) Emission spectrum of Au(I):6-thiogunosine hydrogel with excitation 360 nm, and(b) emission spectrum Au(I):6-thioguanosine hydrogel carrying AuNPs with excitation 360 nm.

excitation and energy transfer from AuNPs to the molecules of hydrogel [32]. Quenching of fluorescence by NPs is effective in sensing/bioimaging especially when the quantum yield is low [33e36]. 3.5. Atomic force microscopy (AFM) studies 3.5.1. The effect of AuNPs on altering the morphology of the host polymer As mentioned in previous literature [18], probing the morphology of Au:6-Tthioguanosine hydrogel using atomic force microscopy (AFM) imaging by depositing 2 mL of the gel onto a silicon wafer (1  1 cm2) followed by air drying revealed formation one dimensional fibres extending many microns in length with

heights ~ 2 nm, Fig. 5(a). While investigation the diameter of the fibres showed that the diameter was 3 nm, with the presence of some bars with higher diameter in the range of 4e7 nm but with less frequency, and some were with lower diameter up to 2 nm but with less frequency, also, as shown in Fig. 5(b). Forming larger diameter fibres can be assigned to intertwine some individual fibres throw entangle the strands of the fibres to form the three dimensional network that requires to form the gel. Atomic force microscopy (AFM) imaging the morphology of the gel that composed AuNPs revealed changing the fabric structure of the gel by the effect of AuNPs. The images showed AuNPs as guest molecules between fibrous chains of Au(I):6-thioguanosine and forming a star-like shape, as shown in Fig. 6(a). The profile in Fig. 6(c), which is associated with the red sloping line across image

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Fig. 5. Tapping mode AFM images of Au(I):6-thioguanosine hydrogel drop-cast onto a silicon wafer and dried in air. The scale bar in (a) is 1 mm, b) The histogram shows the size distribution of the fibres [18].

Fig. 6. (a)Tapping mode AFM image for Au:6-thioguanosing with AuNPs (a) shows the new morphology of the polymer after adding AuNPs. (b) A small area of (a). (c) The profile corresponding to the red sloping line across image (b) displays the heights of AuNPs ~ 8e11 nm. The pitch between the alternating AuNPs was ~300 nm.

(b), displays the pitch between the alternating AuNPs (~300 nm). Probing the heights of these AuNPs in the gel displayed that their heights were in the range of 8e15 nm, Figs. 6(c) & Fig. 7(b). While the height of the new fibre was around 1.5 nm, as shown in Fig. 7 image (c). This work showed with more clearly that the morphology of the Au(I) gel can be turned into new conformation

by composing with AuNPs. Changing the morphology of materials upon using NPs was reported by several researchers, for example, Li and et al., used p-p interaction between ssDNA and reduced graphene oxide that used for synthesis cotton-flower-like by electrodepositing platinum nanoparticles (PtNPs) [37]. Another work by Gupta and et al. displayed the effect of inorganic nanoparticles (iron

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Fig. 7. (a) Tapping mode AFM image for Au:6-thioguanosing with AuNPs. (b) The profile corresponding to the three coloured lines across image (a) displays height fibre ~ 1.5 nm (green & red colours), and height of AuNPs ~ 15 nm (blue colour) which appears as white protrude balls, respectively. (c) The diameter of the fibres in image (a), the main diameter was 1.5 nm, and the average was 2.25 nm.

oxide, CoFe2O4, and Au) on the self-assembling of bacteriophage P22 via electrostatic interaction [38]. Xu and et al. demonstrated that ligand-receptor interaction can affect the formation of various nanostructures of molecular assemblies formed by enzymeinstructed self-assembly of small peptide molecules [39].

these entities. Based on these findings, the host-guest chemistry system for Au(I) hydrogel and AuNPs can provide a good platform for using this important class of nanomaterials in different applications ranging from biomolecule recognition, nanotechnology, drug delivery, etc.

3.5.2. Effect of AuNPs on the diameter of the fibres Statistical analysis was carried out to estimate the diameter of the new polymer that produced with AuNPs. Fig. 7(a) shows AFM image that used for the analysis, Fig. 7(b) displays the profiles of the three sloping lines across image (a), the blue colour curve corresponding to the height of the AuNPs which found around ~ 15 nm while the green and the red colour curves refer to the height fibre in image (a) which was ~1.5 nm. Obtained results in Fig. 7(c) revealed formation an ideal diameter (1.5 nm) for the hydrogel that carrying AuNPs, the data showed dramatic decline in the diameter value compared to the diameter value of the hydrogel without AuNPs (3 nm). The decrease in the fibre diameter upon treating with AuNP could be assigned to increase the electrical conductivity of the fibre which leads to enhance the features of the fibre such as an electrical charge density, large surface area, and high porosity, and as a consequent, obtaining highly attractive polymer characterise with large specific surface area, high porosity, and better interconnectivity that can be used in different applications [12,16,40,41].

Acknowledgements

4. Conclusion In summary, the linear fabric structure for Au(I) 6-thioguanosine can alert to new morphology by combining with AuNPs via intramolecular interactions. The results showed formation typical diameter (~1.5 nm) for the new fabric structure that composed AuNPs compared with the diameter of the hydrogel without NPs (3 nm). Furthermore, hydrogel combining AuNPs revealed quenching the fluorescence properties which can be assigned to the dipole-dipole interactions between NPs and the molecules of the hydrogel, in addition, to the effect of the close distances between

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